Fix drf-spectacular schema generation warnings

[krmax44]

We get a lot of warnings when generating OpenAPI schema with drf-spectacular:

Schema generation summary:
Warnings: 85 (77 unique)
Errors:   0 (0 unique)

Example:

Warning [FoiAttachmentViewSet > FoiAttachmentSerializer]: unable to resolve type hint for function "is_image". Consider using a type hint or @extend_schema_field. Defaulting to string.

Adding some type hints and applying @extend_schema should to the trick.