My goal is to upload a file together with metadata (documentIds) using a multipart request. I am creating the client with the Java OpenAPI generator and the library resttemplate for the following API spec:

path:
  /files:
    post:
      ...
      requestBody:
        content:
          multipart/form-data:
            encoding:
              file:
                contentType: multipart/form-data
                style: form
              documentIds:
                contentType: application/json
                style: form
            schema:
              $ref: '#/components/schemas/UploadFileRequest'
...
components:
  schemas:
    UploadFileRequest:
      type: object
      properties:
        documentIds:
          type: array
          items:
            type: string
        file:
          type: string
          format: binary

The following method is created as a result of the generation:

public void postFile(List<String> documentIds, Resource file) {...}

The problem I am facing is that the content-type for documentIds is automatically set to text/plain which is not accepted on the server side. I couldn’t find any option to define the content-type header for a part. I have tried to define it in the API specification using encoding (see above) and also with the extension x-content-type. Both options didn’t change anything on client side. Am I missing something or is this “feature” just not supported by the library?