diff --git a/.changeset/fix_nullable_and_required_properties_in_multipart_bodies.md b/.changeset/fix_nullable_and_required_properties_in_multipart_bodies.md
new file mode 100644
index 000000000..06c4ea12e
--- /dev/null
+++ b/.changeset/fix_nullable_and_required_properties_in_multipart_bodies.md
@@ -0,0 +1,10 @@
+---
+default: patch
+---
+
+# Fix nullable and required properties in multipart bodies
+
+Fixes #926.
+
+> [!WARNING]
+> This change is likely to break custom templates. Multipart body handling has been completely split from JSON bodies.
diff --git a/end_to_end_tests/baseline_openapi_3.0.json b/end_to_end_tests/baseline_openapi_3.0.json
index 14fdd7c42..11308a286 100644
--- a/end_to_end_tests/baseline_openapi_3.0.json
+++ b/end_to_end_tests/baseline_openapi_3.0.json
@@ -1415,7 +1415,9 @@
     },
     "/naming/mixed-case": {
       "get": {
-        "tags": ["naming"],
+        "tags": [
+          "naming"
+        ],
         "operationId": "mixed_case",
         "parameters": [
           {
@@ -1436,30 +1438,32 @@
           }
         ],
         "responses": {
-            "200": {
-              "description": "Successful response",
-              "content": {
-                "application/json": {
-                  "schema": {
-                    "type": "object",
-                    "properties": {
-                      "mixed_case": {
-                        "type": "string"
-                      },
-                      "mixedCase": {
-                        "type": "string"
-                      }
+          "200": {
+            "description": "Successful response",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "object",
+                  "properties": {
+                    "mixed_case": {
+                      "type": "string"
+                    },
+                    "mixedCase": {
+                      "type": "string"
                     }
                   }
                 }
               }
             }
+          }
         }
       }
     },
     "/naming/{hyphen-in-path}": {
       "get": {
-        "tags": ["naming"],
+        "tags": [
+          "naming"
+        ],
         "operationId": "hyphen_in_path",
         "parameters": [
           {
@@ -1863,7 +1867,8 @@
         "required": [
           "some_file",
           "some_object",
-          "some_nullable_object"
+          "some_nullable_object",
+          "some_required_number"
         ],
         "type": "object",
         "properties": {
@@ -1896,6 +1901,15 @@
             "title": "Some Number",
             "type": "number"
           },
+          "some_nullable_number": {
+            "title": "Some Nullable Number",
+            "type": "number",
+            "nullable": true
+          },
+          "some_required_number": {
+            "title": "Some Required Number",
+            "type": "number"
+          },
           "some_array": {
             "title": "Some Array",
             "nullable": true,
diff --git a/end_to_end_tests/baseline_openapi_3.1.yaml b/end_to_end_tests/baseline_openapi_3.1.yaml
index a0e762a95..39a990db2 100644
--- a/end_to_end_tests/baseline_openapi_3.1.yaml
+++ b/end_to_end_tests/baseline_openapi_3.1.yaml
@@ -1877,7 +1877,8 @@ info:
       "required": [
         "some_file",
         "some_object",
-        "some_nullable_object"
+        "some_nullable_object",
+        "some_required_number"
       ],
       "type": "object",
       "properties": {
@@ -1910,6 +1911,14 @@ info:
           "title": "Some Number",
           "type": "number"
         },
+        "some_nullable_number": {
+          "title": "Some Nullable Number",
+          "type": [ "number", "null" ]
+        },
+        "some_required_number": {
+          "title": "Some Number",
+          "type": "number"
+        },
         "some_array": {
           "title": "Some Array",
           "type": [ "array", "null" ],
diff --git a/end_to_end_tests/golden-record/my_test_api_client/models/body_upload_file_tests_upload_post.py b/end_to_end_tests/golden-record/my_test_api_client/models/body_upload_file_tests_upload_post.py
index 20d27d4a6..aa36dba04 100644
--- a/end_to_end_tests/golden-record/my_test_api_client/models/body_upload_file_tests_upload_post.py
+++ b/end_to_end_tests/golden-record/my_test_api_client/models/body_upload_file_tests_upload_post.py
@@ -32,6 +32,7 @@ class BodyUploadFileTestsUploadPost:
     """
     Attributes:
         some_file (File):
+        some_required_number (float):
         some_object (BodyUploadFileTestsUploadPostSomeObject):
         some_nullable_object (Union['BodyUploadFileTestsUploadPostSomeNullableObject', None]):
         some_optional_file (Union[Unset, File]):
@@ -39,12 +40,14 @@ class BodyUploadFileTestsUploadPost:
         a_datetime (Union[Unset, datetime.datetime]):
         a_date (Union[Unset, datetime.date]):
         some_number (Union[Unset, float]):
+        some_nullable_number (Union[None, Unset, float]):
         some_array (Union[List['AFormData'], None, Unset]):
         some_optional_object (Union[Unset, BodyUploadFileTestsUploadPostSomeOptionalObject]):
         some_enum (Union[Unset, DifferentEnum]): An enumeration.
     """
 
     some_file: File
+    some_required_number: float
     some_object: "BodyUploadFileTestsUploadPostSomeObject"
     some_nullable_object: Union["BodyUploadFileTestsUploadPostSomeNullableObject", None]
     some_optional_file: Union[Unset, File] = UNSET
@@ -52,6 +55,7 @@ class BodyUploadFileTestsUploadPost:
     a_datetime: Union[Unset, datetime.datetime] = UNSET
     a_date: Union[Unset, datetime.date] = UNSET
     some_number: Union[Unset, float] = UNSET
+    some_nullable_number: Union[None, Unset, float] = UNSET
     some_array: Union[List["AFormData"], None, Unset] = UNSET
     some_optional_object: Union[Unset, "BodyUploadFileTestsUploadPostSomeOptionalObject"] = UNSET
     some_enum: Union[Unset, DifferentEnum] = UNSET
@@ -66,6 +70,8 @@ def to_dict(self) -> Dict[str, Any]:
 
         some_file = self.some_file.to_tuple()
 
+        some_required_number = self.some_required_number
+
         some_object = self.some_object.to_dict()
 
         some_nullable_object: Union[Dict[str, Any], None]
@@ -90,6 +96,12 @@ def to_dict(self) -> Dict[str, Any]:
 
         some_number = self.some_number
 
+        some_nullable_number: Union[None, Unset, float]
+        if isinstance(self.some_nullable_number, Unset):
+            some_nullable_number = UNSET
+        else:
+            some_nullable_number = self.some_nullable_number
+
         some_array: Union[List[Dict[str, Any]], None, Unset]
         if isinstance(self.some_array, Unset):
             some_array = UNSET
@@ -116,6 +128,7 @@ def to_dict(self) -> Dict[str, Any]:
         field_dict.update(
             {
                 "some_file": some_file,
+                "some_required_number": some_required_number,
                 "some_object": some_object,
                 "some_nullable_object": some_nullable_object,
             }
@@ -130,6 +143,8 @@ def to_dict(self) -> Dict[str, Any]:
             field_dict["a_date"] = a_date
         if some_number is not UNSET:
             field_dict["some_number"] = some_number
+        if some_nullable_number is not UNSET:
+            field_dict["some_nullable_number"] = some_nullable_number
         if some_array is not UNSET:
             field_dict["some_array"] = some_array
         if some_optional_object is not UNSET:
@@ -142,13 +157,16 @@ def to_dict(self) -> Dict[str, Any]:
     def to_multipart(self) -> Dict[str, Any]:
         some_file = self.some_file.to_tuple()
 
+        some_required_number = (None, str(self.some_required_number).encode(), "text/plain")
+
         some_object = (None, json.dumps(self.some_object.to_dict()).encode(), "application/json")
 
-        some_nullable_object: Union[None, Tuple[None, bytes, str]]
+        some_nullable_object: Tuple[None, bytes, str]
+
         if isinstance(self.some_nullable_object, BodyUploadFileTestsUploadPostSomeNullableObject):
             some_nullable_object = (None, json.dumps(self.some_nullable_object.to_dict()).encode(), "application/json")
         else:
-            some_nullable_object = self.some_nullable_object
+            some_nullable_object = (None, str(self.some_nullable_object).encode(), "text/plain")
 
         some_optional_file: Union[Unset, FileJsonType] = UNSET
         if not isinstance(self.some_optional_file, Unset):
@@ -174,7 +192,17 @@ def to_multipart(self) -> Dict[str, Any]:
             else (None, str(self.some_number).encode(), "text/plain")
         )
 
-        some_array: Union[None, Tuple[None, bytes, str], Unset]
+        some_nullable_number: Union[Tuple[None, bytes, str], Unset]
+
+        if isinstance(self.some_nullable_number, Unset):
+            some_nullable_number = UNSET
+        elif isinstance(self.some_nullable_number, float):
+            some_nullable_number = (None, str(self.some_nullable_number).encode(), "text/plain")
+        else:
+            some_nullable_number = (None, str(self.some_nullable_number).encode(), "text/plain")
+
+        some_array: Union[Tuple[None, bytes, str], Unset]
+
         if isinstance(self.some_array, Unset):
             some_array = UNSET
         elif isinstance(self.some_array, list):
@@ -183,9 +211,8 @@ def to_multipart(self) -> Dict[str, Any]:
                 some_array_type_0_item = some_array_type_0_item_data.to_dict()
                 _temp_some_array.append(some_array_type_0_item)
             some_array = (None, json.dumps(_temp_some_array).encode(), "application/json")
-
         else:
-            some_array = self.some_array
+            some_array = (None, str(self.some_array).encode(), "text/plain")
 
         some_optional_object: Union[Unset, Tuple[None, bytes, str]] = UNSET
         if not isinstance(self.some_optional_object, Unset):
@@ -201,6 +228,7 @@ def to_multipart(self) -> Dict[str, Any]:
         field_dict.update(
             {
                 "some_file": some_file,
+                "some_required_number": some_required_number,
                 "some_object": some_object,
                 "some_nullable_object": some_nullable_object,
             }
@@ -215,6 +243,8 @@ def to_multipart(self) -> Dict[str, Any]:
             field_dict["a_date"] = a_date
         if some_number is not UNSET:
             field_dict["some_number"] = some_number
+        if some_nullable_number is not UNSET:
+            field_dict["some_nullable_number"] = some_nullable_number
         if some_array is not UNSET:
             field_dict["some_array"] = some_array
         if some_optional_object is not UNSET:
@@ -241,6 +271,8 @@ def from_dict(cls: Type[T], src_dict: Dict[str, Any]) -> T:
         d = src_dict.copy()
         some_file = File(payload=BytesIO(d.pop("some_file")))
 
+        some_required_number = d.pop("some_required_number")
+
         some_object = BodyUploadFileTestsUploadPostSomeObject.from_dict(d.pop("some_object"))
 
         def _parse_some_nullable_object(data: object) -> Union["BodyUploadFileTestsUploadPostSomeNullableObject", None]:
@@ -283,6 +315,15 @@ def _parse_some_nullable_object(data: object) -> Union["BodyUploadFileTestsUploa
 
         some_number = d.pop("some_number", UNSET)
 
+        def _parse_some_nullable_number(data: object) -> Union[None, Unset, float]:
+            if data is None:
+                return data
+            if isinstance(data, Unset):
+                return data
+            return cast(Union[None, Unset, float], data)
+
+        some_nullable_number = _parse_some_nullable_number(d.pop("some_nullable_number", UNSET))
+
         def _parse_some_array(data: object) -> Union[List["AFormData"], None, Unset]:
             if data is None:
                 return data
@@ -321,6 +362,7 @@ def _parse_some_array(data: object) -> Union[List["AFormData"], None, Unset]:
 
         body_upload_file_tests_upload_post = cls(
             some_file=some_file,
+            some_required_number=some_required_number,
             some_object=some_object,
             some_nullable_object=some_nullable_object,
             some_optional_file=some_optional_file,
@@ -328,6 +370,7 @@ def _parse_some_array(data: object) -> Union[List["AFormData"], None, Unset]:
             a_datetime=a_datetime,
             a_date=a_date,
             some_number=some_number,
+            some_nullable_number=some_nullable_number,
             some_array=some_array,
             some_optional_object=some_optional_object,
             some_enum=some_enum,
diff --git a/end_to_end_tests/golden-record/my_test_api_client/models/post_bodies_multiple_files_body.py b/end_to_end_tests/golden-record/my_test_api_client/models/post_bodies_multiple_files_body.py
index 1c61d3385..c81dc7636 100644
--- a/end_to_end_tests/golden-record/my_test_api_client/models/post_bodies_multiple_files_body.py
+++ b/end_to_end_tests/golden-record/my_test_api_client/models/post_bodies_multiple_files_body.py
@@ -33,9 +33,9 @@ def to_multipart(self) -> Dict[str, Any]:
         a = self.a if isinstance(self.a, Unset) else (None, str(self.a).encode(), "text/plain")
 
         field_dict: Dict[str, Any] = {}
-        field_dict.update(
-            {key: (None, str(value).encode(), "text/plain") for key, value in self.additional_properties.items()}
-        )
+        for prop_name, prop in self.additional_properties.items():
+            field_dict[prop_name] = (None, str(prop).encode(), "text/plain")
+
         field_dict.update({})
         if a is not UNSET:
             field_dict["a"] = a
diff --git a/integration-tests/integration_tests/models/post_body_multipart_body.py b/integration-tests/integration_tests/models/post_body_multipart_body.py
index de9992232..f0272a34e 100644
--- a/integration-tests/integration_tests/models/post_body_multipart_body.py
+++ b/integration-tests/integration_tests/models/post_body_multipart_body.py
@@ -45,9 +45,7 @@ def to_dict(self) -> Dict[str, Any]:
         return field_dict
 
     def to_multipart(self) -> Dict[str, Any]:
-        a_string = (
-            self.a_string if isinstance(self.a_string, Unset) else (None, str(self.a_string).encode(), "text/plain")
-        )
+        a_string = (None, str(self.a_string).encode(), "text/plain")
 
         file = self.file.to_tuple()
 
@@ -58,9 +56,9 @@ def to_multipart(self) -> Dict[str, Any]:
         )
 
         field_dict: Dict[str, Any] = {}
-        field_dict.update(
-            {key: (None, str(value).encode(), "text/plain") for key, value in self.additional_properties.items()}
-        )
+        for prop_name, prop in self.additional_properties.items():
+            field_dict[prop_name] = (None, str(prop).encode(), "text/plain")
+
         field_dict.update(
             {
                 "a_string": a_string,
diff --git a/openapi_python_client/parser/properties/model_property.py b/openapi_python_client/parser/properties/model_property.py
index bde45ac05..81734e461 100644
--- a/openapi_python_client/parser/properties/model_property.py
+++ b/openapi_python_client/parser/properties/model_property.py
@@ -7,7 +7,9 @@
 
 from ... import Config, utils
 from ... import schema as oai
+from ...utils import PythonIdentifier
 from ..errors import ParseError, PropertyError
+from .any import AnyProperty
 from .enum_property import EnumProperty
 from .protocol import PropertyProtocol, Value
 from .schemas import Class, ReferencePath, Schemas, parse_reference_path
@@ -30,7 +32,7 @@ class ModelProperty(PropertyProtocol):
     optional_properties: list[Property] | None
     relative_imports: set[str] | None
     lazy_imports: set[str] | None
-    additional_properties: bool | Property | None
+    additional_properties: Property | None
     _json_type_string: ClassVar[str] = "Dict[str, Any]"
 
     template: ClassVar[str] = "model_property.py.jinja"
@@ -78,7 +80,7 @@ def build(
         optional_properties: list[Property] | None = None
         relative_imports: set[str] | None = None
         lazy_imports: set[str] | None = None
-        additional_properties: bool | Property | None = None
+        additional_properties: Property | None = None
         if process_properties:
             data_or_err, schemas = _process_property_data(
                 data=data, schemas=schemas, class_info=class_info, config=config, roots=model_roots
@@ -386,6 +388,16 @@ def _add_if_no_conflict(new_prop: Property) -> PropertyError | None:
     )
 
 
+ANY_ADDITIONAL_PROPERTY = AnyProperty.build(
+    name="additional",
+    required=True,
+    default=None,
+    description="",
+    python_name=PythonIdentifier(value="additional", prefix=""),
+    example=None,
+)
+
+
 def _get_additional_properties(
     *,
     schema_additional: None | (bool | (oai.Reference | oai.Schema)),
@@ -393,18 +405,20 @@ def _get_additional_properties(
     class_name: utils.ClassName,
     config: Config,
     roots: set[ReferencePath | utils.ClassName],
-) -> tuple[bool | (Property | PropertyError), Schemas]:
+) -> tuple[Property | None | PropertyError, Schemas]:
     from . import property_from_data
 
     if schema_additional is None:
-        return True, schemas
+        return ANY_ADDITIONAL_PROPERTY, schemas
 
     if isinstance(schema_additional, bool):
-        return schema_additional, schemas
+        if schema_additional:
+            return ANY_ADDITIONAL_PROPERTY, schemas
+        return None, schemas
 
     if isinstance(schema_additional, oai.Schema) and not any(schema_additional.model_dump().values()):
         # An empty schema
-        return True, schemas
+        return ANY_ADDITIONAL_PROPERTY, schemas
 
     additional_properties, schemas = property_from_data(
         name="AdditionalProperty",
@@ -425,7 +439,7 @@ def _process_property_data(
     class_info: Class,
     config: Config,
     roots: set[ReferencePath | utils.ClassName],
-) -> tuple[tuple[_PropertyData, bool | Property] | PropertyError, Schemas]:
+) -> tuple[tuple[_PropertyData, Property | None] | PropertyError, Schemas]:
     property_data = _process_properties(
         data=data, schemas=schemas, class_name=class_info.name, config=config, roots=roots
     )
@@ -442,7 +456,7 @@ def _process_property_data(
     )
     if isinstance(additional_properties, PropertyError):
         return additional_properties, schemas
-    elif isinstance(additional_properties, bool):
+    elif additional_properties is None:
         pass
     else:
         property_data.relative_imports.update(additional_properties.get_imports(prefix=".."))
diff --git a/openapi_python_client/parser/properties/protocol.py b/openapi_python_client/parser/properties/protocol.py
index b8237923d..e3f2ceaac 100644
--- a/openapi_python_client/parser/properties/protocol.py
+++ b/openapi_python_client/parser/properties/protocol.py
@@ -107,6 +107,8 @@ def get_type_string(
         """
         if json:
             type_string = self.get_base_json_type_string(quoted=quoted)
+        elif multipart:
+            type_string = "Tuple[None, bytes, str]"
         else:
             type_string = self.get_base_type_string(quoted=quoted)
 
diff --git a/openapi_python_client/templates/endpoint_macros.py.jinja b/openapi_python_client/templates/endpoint_macros.py.jinja
index da02b5c4a..79ca33d57 100644
--- a/openapi_python_client/templates/endpoint_macros.py.jinja
+++ b/openapi_python_client/templates/endpoint_macros.py.jinja
@@ -83,8 +83,8 @@ params = {k: v for k, v in params.items() if v is not UNSET and v is not None}
 {% macro multipart_body(body, destination) %}
 {% set property = body.prop %}
 {% import "property_templates/" + property.template as prop_template %}
-{% if prop_template.transform_multipart %}
-{{ prop_template.transform_multipart(property, property.python_name, destination) }}
+{% if prop_template.transform_multipart_body %}
+{{ prop_template.transform_multipart_body(property, property.python_name, destination) }}
 {% endif %}
 {% endmacro %}
 
diff --git a/openapi_python_client/templates/model.py.jinja b/openapi_python_client/templates/model.py.jinja
index 0df641ea4..44f5bf148 100644
--- a/openapi_python_client/templates/model.py.jinja
+++ b/openapi_python_client/templates/model.py.jinja
@@ -80,10 +80,10 @@ class {{ class_name }}:
 {% macro _to_dict(multipart=False) %}
 {% for property in model.required_properties + model.optional_properties %}
 {% import "property_templates/" + property.template as prop_template %}
-{% if prop_template.transform %}
-{{ prop_template.transform(property, "self." + property.python_name, property.python_name, multipart=multipart) }}
-{% elif multipart %}
-{{ property.python_name }} = self.{{ property.python_name }} if isinstance(self.{{ property.python_name }}, Unset) else (None, str(self.{{ property.python_name }}).encode(), "text/plain")
+{% if multipart %}
+{{ prop_template.transform_multipart(property, "self." + property.python_name, property.python_name) }}
+{% elif prop_template.transform %}
+{{ prop_template.transform(property=property, source="self." + property.python_name, destination=property.python_name) }}
 {% else %}
 {{ property.python_name }} = self.{{ property.python_name }}
 {% endif %}
@@ -92,19 +92,13 @@ class {{ class_name }}:
 
 field_dict: Dict[str, Any] = {}
 {% if model.additional_properties %}
-{% if model.additional_properties.template %}{# Can be a bool instead of an object #}
-    {% import "property_templates/" + model.additional_properties.template as prop_template %}
-{% else %}
-    {% set prop_template = None %}
-{% endif %}
-{% if prop_template and prop_template.transform %}
+{% import "property_templates/" + model.additional_properties.template as prop_template %}
+{% if multipart %}
 for prop_name, prop in self.additional_properties.items():
-    {{ prop_template.transform(model.additional_properties, "prop", "field_dict[prop_name]", multipart=multipart, declare_type=false) | indent(4) }}
-{% elif multipart %}
-field_dict.update({
-    key: (None, str(value).encode(), "text/plain")
-    for key, value in self.additional_properties.items()
-})
+    {{ prop_template.transform_multipart(model.additional_properties, "prop", "field_dict[prop_name]") | indent(4) }}
+{% elif prop_template.transform %}
+for prop_name, prop in self.additional_properties.items():
+    {{ prop_template.transform(model.additional_properties, "prop", "field_dict[prop_name]", declare_type=false) | indent(4) }}
 {% else %}
 field_dict.update(self.additional_properties)
 {% endif %}
diff --git a/openapi_python_client/templates/property_templates/any_property.py.jinja b/openapi_python_client/templates/property_templates/any_property.py.jinja
index e69de29bb..25a16516a 100644
--- a/openapi_python_client/templates/property_templates/any_property.py.jinja
+++ b/openapi_python_client/templates/property_templates/any_property.py.jinja
@@ -0,0 +1,7 @@
+{% macro transform_multipart(property, source, destination) %}
+{% if not property.required %}
+{{ destination }} = {{source}} if isinstance({{source}}, Unset) else (None, str({{source}}).encode(), "text/plain")
+{% else %}
+{{ destination }} =  (None, str({{ source }}).encode(), "text/plain")
+{% endif %}
+{% endmacro %}
\ No newline at end of file
diff --git a/openapi_python_client/templates/property_templates/boolean_property.py.jinja b/openapi_python_client/templates/property_templates/boolean_property.py.jinja
index 3b16b7d20..52fda08e1 100644
--- a/openapi_python_client/templates/property_templates/boolean_property.py.jinja
+++ b/openapi_python_client/templates/property_templates/boolean_property.py.jinja
@@ -1,3 +1,11 @@
 {% macro transform_header(source) %}
 "true" if {{ source }} else "false"
 {% endmacro %}
+
+{% macro transform_multipart(property, source, destination) %}
+{% if not property.required %}
+{{ destination }} = {{source}} if isinstance({{source}}, Unset) else (None, str({{source}}).encode(), "text/plain")
+{% else %}
+{{ destination }} =  (None, str({{ source }}).encode(), "text/plain")
+{% endif %}
+{% endmacro %}
diff --git a/openapi_python_client/templates/property_templates/date_property.py.jinja b/openapi_python_client/templates/property_templates/date_property.py.jinja
index 5a3fdeafc..b5da665f7 100644
--- a/openapi_python_client/templates/property_templates/date_property.py.jinja
+++ b/openapi_python_client/templates/property_templates/date_property.py.jinja
@@ -10,17 +10,13 @@ isoparse({{ source }}).date()
 
 {% macro check_type_for_construct(property, source) %}isinstance({{ source }}, str){% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True, multipart=False) %}
+{% macro transform(property, source, destination, declare_type=True) %}
 {% set transformed = source + ".isoformat()" %}
-{% if multipart %}{# Multipart data must be bytes, not str #}
-{% set transformed = transformed + ".encode()" %}
-{% endif %}
 {% if property.required %}
 {{ destination }} = {{ transformed }}
 {%- else %}
 {% if declare_type %}
 {% set type_annotation = property.get_type_string(json=True) %}
-{% if multipart %}{% set type_annotation = type_annotation | replace("str", "bytes") %}{% endif %}
 {{ destination }}: {{ type_annotation }} = UNSET
 {% else %}
 {{ destination }} = UNSET
@@ -29,3 +25,15 @@ if not isinstance({{ source }}, Unset):
     {{ destination }} = {{ transformed }}
 {%- endif %}
 {% endmacro %}
+
+{% macro transform_multipart(property, source, destination) %}
+{% set transformed = source + ".isoformat().encode()" %}
+{% if property.required %}
+{{ destination }} = {{ transformed }}
+{%- else %}
+{% set type_annotation = property.get_type_string(json=True) | replace("str", "bytes") %}
+{{ destination }}: {{ type_annotation }} = UNSET
+if not isinstance({{ source }}, Unset):
+    {{ destination }} = {{ transformed }}
+{%- endif %}
+{% endmacro %}
diff --git a/openapi_python_client/templates/property_templates/datetime_property.py.jinja b/openapi_python_client/templates/property_templates/datetime_property.py.jinja
index 2ff54f4dc..32e3f2ee6 100644
--- a/openapi_python_client/templates/property_templates/datetime_property.py.jinja
+++ b/openapi_python_client/templates/property_templates/datetime_property.py.jinja
@@ -10,17 +10,13 @@ isoparse({{ source }})
 
 {% macro check_type_for_construct(property, source) %}isinstance({{ source }}, str){% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True, multipart=False) %}
+{% macro transform(property, source, destination, declare_type=True) %}
 {% set transformed = source + ".isoformat()" %}
-{% if multipart %}{# Multipart data must be bytes, not str #}
-{% set transformed = transformed + ".encode()" %}
-{% endif %}
 {% if property.required %}
 {{ destination }} = {{ transformed }}
 {%- else %}
 {% if declare_type %}
 {% set type_annotation = property.get_type_string(json=True) %}
-{% if multipart %}{% set type_annotation = type_annotation | replace("str", "bytes") %}{% endif %}
 {{ destination }}: {{ type_annotation }} = UNSET
 {% else %}
 {{ destination }} = UNSET
@@ -29,3 +25,15 @@ if not isinstance({{ source }}, Unset):
     {{ destination }} = {{ transformed }}
 {%- endif %}
 {% endmacro %}
+
+{% macro transform_multipart(property, source, destination) %}
+{% set transformed = source + ".isoformat().encode()" %}
+{% if property.required %}
+{{ destination }} = {{ transformed }}
+{%- else %}
+{% set type_annotation = property.get_type_string(json=True) | replace("str", "bytes") %}
+{{ destination }}: {{ type_annotation }} = UNSET
+if not isinstance({{ source }}, Unset):
+    {{ destination }} = {{ transformed }}
+{%- endif %}
+{% endmacro %}
diff --git a/openapi_python_client/templates/property_templates/enum_property.py.jinja b/openapi_python_client/templates/property_templates/enum_property.py.jinja
index d01137f03..ea9b66a51 100644
--- a/openapi_python_client/templates/property_templates/enum_property.py.jinja
+++ b/openapi_python_client/templates/property_templates/enum_property.py.jinja
@@ -13,10 +13,6 @@
 {% macro transform(property, source, destination, declare_type=True, multipart=False) %}
 {% set transformed = source + ".value" %}
 {% set type_string = property.get_type_string(json=True) %}
-{% if multipart %}
-    {% set transformed = "(None, str(" + transformed + ").encode(), \"text/plain\")" %}
-    {% set type_string = "Union[Unset, Tuple[None, bytes, str]]" %}
-{% endif %}
 {% if property.required %}
 {{ destination }} = {{ transformed }}
 {%- else %}
@@ -26,6 +22,18 @@ if not isinstance({{ source }}, Unset):
 {% endif %}
 {% endmacro %}
 
+{% macro transform_multipart(property, source, destination) %}
+{% set transformed = "(None, str(" + source + ".value" + ").encode(), \"text/plain\")" %}
+{% set type_string = "Union[Unset, Tuple[None, bytes, str]]" %}
+{% if property.required %}
+{{ destination }} = {{ transformed }}
+{%- else %}
+{{ destination }}: {{ type_string }} = UNSET
+if not isinstance({{ source }}, Unset):
+    {{ destination }} = {{ transformed }}
+{% endif %}
+{% endmacro %}
+
 {% macro transform_header(source) %}
 str({{ source }})
 {% endmacro %}
diff --git a/openapi_python_client/templates/property_templates/file_property.py.jinja b/openapi_python_client/templates/property_templates/file_property.py.jinja
index c19a068c5..8d27ae617 100644
--- a/openapi_python_client/templates/property_templates/file_property.py.jinja
+++ b/openapi_python_client/templates/property_templates/file_property.py.jinja
@@ -12,7 +12,7 @@ File(
 
 {% macro check_type_for_construct(property, source) %}isinstance({{ source }}, bytes){% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True, multipart=False) %}
+{% macro transform(property, source, destination, declare_type=True) %}
 {% if property.required %}
 {{ destination }} = {{ source }}.to_tuple()
 {% else %}
@@ -21,3 +21,13 @@ if not isinstance({{ source }}, Unset):
     {{ destination }} = {{ source }}.to_tuple()
 {% endif %}
 {% endmacro %}
+
+{% macro transform_multipart(property, source, destination) %}
+{% if property.required %}
+{{ destination }} = {{ source }}.to_tuple()
+{% else %}
+{{ destination }}: {{ property.get_type_string(json=True) }} = UNSET
+if not isinstance({{ source }}, Unset):
+    {{ destination }} = {{ source }}.to_tuple()
+{% endif %}
+{% endmacro %}
diff --git a/openapi_python_client/templates/property_templates/float_property.py.jinja b/openapi_python_client/templates/property_templates/float_property.py.jinja
index 0d433c22e..12ffb0fb4 100644
--- a/openapi_python_client/templates/property_templates/float_property.py.jinja
+++ b/openapi_python_client/templates/property_templates/float_property.py.jinja
@@ -1,3 +1,11 @@
 {% macro transform_header(source) %}
 str({{ source }})
 {% endmacro %}
+
+{% macro transform_multipart(property, source, destination) %}
+{% if not property.required %}
+{{ destination }} = {{source}} if isinstance({{source}}, Unset) else (None, str({{source}}).encode(), "text/plain")
+{% else %}
+{{ destination }} =  (None, str({{ source }}).encode(), "text/plain")
+{% endif %}
+{% endmacro %}
diff --git a/openapi_python_client/templates/property_templates/int_property.py.jinja b/openapi_python_client/templates/property_templates/int_property.py.jinja
index 0d433c22e..12ffb0fb4 100644
--- a/openapi_python_client/templates/property_templates/int_property.py.jinja
+++ b/openapi_python_client/templates/property_templates/int_property.py.jinja
@@ -1,3 +1,11 @@
 {% macro transform_header(source) %}
 str({{ source }})
 {% endmacro %}
+
+{% macro transform_multipart(property, source, destination) %}
+{% if not property.required %}
+{{ destination }} = {{source}} if isinstance({{source}}, Unset) else (None, str({{source}}).encode(), "text/plain")
+{% else %}
+{{ destination }} =  (None, str({{ source }}).encode(), "text/plain")
+{% endif %}
+{% endmacro %}
diff --git a/openapi_python_client/templates/property_templates/list_property.py.jinja b/openapi_python_client/templates/property_templates/list_property.py.jinja
index 0e5d1b5e3..4b51c8e01 100644
--- a/openapi_python_client/templates/property_templates/list_property.py.jinja
+++ b/openapi_python_client/templates/property_templates/list_property.py.jinja
@@ -40,24 +40,38 @@ for {{ inner_source }} in {{ source }}:
 
 {% macro check_type_for_construct(property, source) %}isinstance({{ source }}, list){% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True, multipart=False, transform_method="to_dict") %}
+{% macro transform(property, source, destination, declare_type=True) %}
 {% set inner_property = property.inner_property %}
-{% if multipart %}
-    {% set type_string = "Union[Unset, Tuple[None, bytes, str]]" %}
-{% else %}
-    {% set type_string = property.get_type_string(json=True) %}
-{% endif %}
+{% set type_string = property.get_type_string(json=True) %}
 {% if property.required %}
-{{ _transform(property, source, destination, multipart, transform_method) }}
+{{ _transform(property, source, destination, False, "to_dict") }}
 {% else %}
 {{ destination }}{% if declare_type %}: {{ type_string }}{% endif %} = UNSET
 if not isinstance({{ source }}, Unset):
-    {{ _transform(property, source, destination, multipart, transform_method) | indent(4)}}
+    {{ _transform(property, source, destination, False, "to_dict") | indent(4)}}
 {% endif %}
-
-
 {% endmacro %}
 
 {% macro transform_multipart(property, source, destination) %}
-{{ transform(property, source, destination, transform_method="to_multipart") }}
+{% set inner_property = property.inner_property %}
+{% set type_string = "Union[Unset, Tuple[None, bytes, str]]" %}
+{% if property.required %}
+{{ _transform(property, source, destination, True, "to_dict") }}
+{% else %}
+{{ destination }}: {{ type_string }} = UNSET
+if not isinstance({{ source }}, Unset):
+{{ _transform(property, source, destination, True, "to_dict") | indent(4)}}
+{% endif %}
+{% endmacro %}
+
+{% macro transform_multipart_body(property, source, destination) %}
+{% set inner_property = property.inner_property %}
+{% set type_string = property.get_type_string(json=True) %}
+{% if property.required %}
+{{ _transform(property, source, destination, False, "to_multipart") }}
+{% else %}
+{{ destination }}: {{ type_string }} = UNSET
+if not isinstance({{ source }}, Unset):
+    {{ _transform(property, source, destination, False, "to_multipart") | indent(4)}}
+{% endif %}
 {% endmacro %}
diff --git a/openapi_python_client/templates/property_templates/model_property.py.jinja b/openapi_python_client/templates/property_templates/model_property.py.jinja
index e7f779563..308b7478b 100644
--- a/openapi_python_client/templates/property_templates/model_property.py.jinja
+++ b/openapi_python_client/templates/property_templates/model_property.py.jinja
@@ -10,14 +10,9 @@
 
 {% macro check_type_for_construct(property, source) %}isinstance({{ source }}, dict){% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True, multipart=False, transform_method="to_dict") %}
-{% set transformed = source + "." + transform_method + "()" %}
-{% if multipart %}
-    {% set transformed = "(None, json.dumps(" + transformed + ").encode(), 'application/json')" %}
-    {% set type_string = property.get_type_string(multipart=True) %}
-{% else %}
-    {% set type_string = property.get_type_string(json=True) %}
-{% endif %}
+{% macro transform(property, source, destination, declare_type=True) %}
+{% set transformed = source + ".to_dict()" %}
+{% set type_string = property.get_type_string(json=True) %}
 {% if property.required %}
 {{ destination }} = {{ transformed }}
 {%- else %}
@@ -27,6 +22,26 @@ if not isinstance({{ source }}, Unset):
 {%- endif %}
 {% endmacro %}
 
+{% macro transform_multipart_body(property, source, destination) %}
+{% set transformed = source + ".to_multipart()" %}
+{% set type_string = property.get_type_string(multipart=True) %}
+{% if property.required %}
+{{ destination }} = {{ transformed }}
+{%- else %}
+{{ destination }}: {{ type_string }} = UNSET
+if not isinstance({{ source }}, Unset):
+    {{ destination }} = {{ transformed }}
+{%- endif %}
+{% endmacro %}
+
 {% macro transform_multipart(property, source, destination) %}
-{{ transform(property, source, destination, transform_method="to_multipart") }}
+{% set transformed = "(None, json.dumps(" + source + ".to_dict()" + ").encode(), 'application/json')" %}
+{% set type_string = property.get_type_string(multipart=True) %}
+{% if property.required %}
+{{ destination }} = {{ transformed }}
+{%- else %}
+{{ destination }}: {{ type_string }} = UNSET
+if not isinstance({{ source }}, Unset):
+    {{ destination }} = {{ transformed }}
+{%- endif %}
 {% endmacro %}
diff --git a/openapi_python_client/templates/property_templates/union_property.py.jinja b/openapi_python_client/templates/property_templates/union_property.py.jinja
index b8ab1962d..dbf7ee9dc 100644
--- a/openapi_python_client/templates/property_templates/union_property.py.jinja
+++ b/openapi_python_client/templates/property_templates/union_property.py.jinja
@@ -39,9 +39,9 @@ def _parse_{{ property.python_name }}(data: object) -> {{ property.get_type_stri
 {{ property.python_name }} = _parse_{{ property.python_name }}({{ source }})
 {% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True, multipart=False) %}
+{% macro transform(property, source, destination, declare_type=True) %}
 {% set ns = namespace(contains_properties_without_transform = false, contains_modified_properties = not property.required, has_if = false) %}
-{% if declare_type %}{{ destination }}: {{ property.get_type_string(json=not multipart, multipart=multipart) }}{% endif %}
+{% if declare_type %}{{ destination }}: {{ property.get_type_string(json=True, multipart=False) }}{% endif %}
 
 {% if not property.required %}
 if isinstance({{ source }}, Unset):
@@ -64,7 +64,7 @@ elif isinstance({{ source }}, {{ inner_property.get_instance_type_string() }}):
     {% else %}
 else:
     {% endif %}
-    {{ inner_template.transform(inner_property, source, destination, declare_type=False, multipart=multipart) | indent(4) }}
+    {{ inner_template.transform(inner_property, source, destination, declare_type=False) | indent(4) }}
 {% endfor %}
 {% if ns.contains_properties_without_transform and ns.contains_modified_properties %}
 else:
@@ -73,3 +73,27 @@ else:
 {{ destination }} = {{ source }}
 {%- endif %}
 {% endmacro %}
+
+
+{% macro transform_multipart(property, source, destination) %}
+{% set ns = namespace(has_if = false) %}
+{{ destination }}: {{ property.get_type_string(json=False, multipart=True) }}
+
+{% if not property.required %}
+if isinstance({{ source }}, Unset):
+    {{ destination }} = UNSET
+    {% set ns.has_if = true %}
+{% endif %}
+{% for inner_property in property.inner_properties %}
+{% if not ns.has_if %}
+if isinstance({{ source }}, {{ inner_property.get_instance_type_string() }}):
+{% set ns.has_if = true %}
+{% elif not loop.last %}
+elif isinstance({{ source }}, {{ inner_property.get_instance_type_string() }}):
+{% else %}
+else:
+{% endif %}
+{% import "property_templates/" + inner_property.template as inner_template %}
+    {{ inner_template.transform_multipart(inner_property, source, destination) | indent(4) | trim }}
+{% endfor %}
+{% endmacro %}
diff --git a/tests/test_parser/test_properties/test_model_property.py b/tests/test_parser/test_properties/test_model_property.py
index 917582042..a7d7c1f23 100644
--- a/tests/test_parser/test_properties/test_model_property.py
+++ b/tests/test_parser/test_properties/test_model_property.py
@@ -6,7 +6,7 @@
 import openapi_python_client.schema as oai
 from openapi_python_client.parser.errors import PropertyError
 from openapi_python_client.parser.properties import Schemas, StringProperty
-from openapi_python_client.parser.properties.model_property import _process_properties
+from openapi_python_client.parser.properties.model_property import ANY_ADDITIONAL_PROPERTY, _process_properties
 
 MODULE_NAME = "openapi_python_client.parser.properties.model_property"
 
@@ -70,10 +70,10 @@ class TestBuild:
     @pytest.mark.parametrize(
         "additional_properties_schema, expected_additional_properties",
         [
-            (True, True),
-            (oai.Schema.model_construct(), True),
-            (None, True),
-            (False, False),
+            (True, ANY_ADDITIONAL_PROPERTY),
+            (oai.Schema.model_construct(), ANY_ADDITIONAL_PROPERTY),
+            (None, ANY_ADDITIONAL_PROPERTY),
+            (False, None),
             (
                 oai.Schema.model_construct(type="string"),
                 StringProperty(
@@ -163,7 +163,7 @@ def test_happy_path(self, model_property_factory, string_property_factory, date_
                 "from typing import Union",
             },
             lazy_imports=set(),
-            additional_properties=True,
+            additional_properties=ANY_ADDITIONAL_PROPERTY,
         )
 
     def test_model_name_conflict(self, config):