[BUG][C#][unityWebRequest] Multipart/form-data request does not attach file and sets Content-Type without boundary

Description

When generating a C# client with --library unityWebRequest for an endpoint that accepts multipart/form-data (string + binary file), the generated code does not attach the file to the request and manually sets Content-Type: multipart/form-data without a boundary. This causes the server (Fastify/busboy) to return:

Multipart: Boundary not found

Expected behavior

• For Unity (unityWebRequest library), the generated code should call WWWForm.AddBinaryData(...) for type: string, format: binary properties.
• The Content-Type header should not be set manually — WWWForm should generate it automatically with the correct boundary.

---

OpenAPI Spec snippet to reproduce

openapi: 3.0.3
info:
  title: Test API
  version: 1.0.0
paths:
  /upload:
    post:
      operationId: uploadFile
      requestBody:
        required: true
        content:
          multipart/form-data:
            schema:
              type: object
              required:
                - raceId
                - file
              properties:
                raceId:
                  type: string
                file:
                  type: string
                  format: binary
      responses:
        '200':
          description: OK

---

Command used for generation

openapi-generator-cli generate   -i swagger.json   -g csharp   --library unityWebRequest   -o sdk-unity/   --skip-validate-spec   --global-property modelTests=false,apiTests=false,modelDocs=false,apiDocs=false,validatable=false   --additional-properties packageName=BX.Rest

---

openapi-generator-cli version

openapi-generator-cli version
# e.g. 7.14.0

---

Actual generated code (excerpt)

string[] _contentTypes = new string[] { "multipart/form-data" };
var localVarContentType = ClientUtils.SelectHeaderContentType(_contentTypes);
if (localVarContentType != null)
    localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);

localVarRequestOptions.FormParameters.Add("raceId", ClientUtils.ParameterToString(raceId));
// No call to FileParameters.Add(...) or WWWForm.AddBinaryData(...)

---

Actual behavior

• File is never added to the multipart request.
• Content-Type header is set manually without a boundary → server throws Multipart: Boundary not found.

---

Expected behavior

• File should be added using:

  form.AddBinaryData(fieldName, fileBytes, fileName, contentType);
  

  for Unity.
• Content-Type should be omitted so that WWWForm generates it with a correct boundary.

---

Additional context

• Works correctly with --library httpclient (file is attached via FileParameters.Add).
• The issue is specific to --library unityWebRequest.

Reproduce the issue with the supplied OpenAPI spec and the unityWebRequest generation command, then trace the generated multipart request handling and its form/header setup. Done means binary properties are included in the Unity form and the manually supplied Content-Type is omitted so the boundary is generated automatically.