fix: Correct status code and message returned on invalid file upload

Author: Ivan-Shaml

apps/api/src/common/exception/file-validation.exception.ts

@@ -0,0 +1,26 @@
+import { HttpStatus, UnsupportedMediaTypeException } from '@nestjs/common'
+
+/**
+ * Custom exception thrown when file validation fails.
+ * This exception provides a structured error response for unsupported or invalid file types.
+ */
+export class FileValidationException extends UnsupportedMediaTypeException {
+  /**
+   * Constructor.
+   *
+   * @param message - The detailed error message describing the validation failure
+   * @param fileName - The name of the file that failed validation
+   */
+  constructor(message: string, fileName = 'file') {
+    super({
+      statusCode: HttpStatus.UNSUPPORTED_MEDIA_TYPE,
+      message: {
+        property: fileName,
+        constraints: {
+          isValidFile: false,
+        },
+      },
+      error: message,
+    })
+  }
+}

apps/api/src/common/files.ts

@@ -1,4 +1,5 @@
 import path from 'path'
+import { FileValidationException } from './exception/file-validation.exception'
 
 interface File {
   fieldname: string
@@ -13,9 +14,11 @@ interface File {
 }
 
 /**
- * The function is used to validate the type of a file using the file extension and MIME type
+ * The function is used to validate the type of file using the file extension and MIME type
  * @param file object that represent file
  * @param cb callback function which indicates whether file is accepted or not
+ *
+ * @throws FileValidationException if the file does not pass validation
  */
 export function validateFileType(
   file: File,
@@ -33,21 +36,27 @@ export function validateFileType(
     'application/vnd.ms-excel',
     'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
   ]
+
+  let filename = file.originalname
+  let extension = path.extname(filename).toLowerCase()
+  if (extension == '') {
+    filename = file.filename
+    // for the expense files, the original filename is encoded in base64
+    extension = path.extname(filename).toLowerCase()
+  }
+
   if (!mimeAllowlist.includes(file.mimetype)) {
-    return cb(new Error('File mime type is not allowed'), false)
+    return cb(new FileValidationException('File mime type is not allowed', filename), false)
   }
 
   const allowedExtensions = /txt|json|pdf|jpeg|jpg|png|xml|xlsx|xls|docx/
 
-  const filename = file.originalname
-  let ext = path.extname(filename).toLowerCase()
-  if (ext == '') {
-    // for the expense files, the original filename is encoded in base64
-    ext = path.extname(file.filename).toLowerCase()
-  }
-  const isExtensionSupported = allowedExtensions.test(ext)
+  const isExtensionSupported = allowedExtensions.test(extension)
   if (!isExtensionSupported) {
-    return cb(new Error('File extension is not allowed: ' + file.filename), false)
+    return cb(
+      new FileValidationException('File extension is not allowed: ' + filename, filename),
+      false,
+    )
   }
 
   cb(null, true)