SomeNetworkOperations.java

public class NetworkOperations {
  
  @Override
    public Response<ResponseBody> directUpload(File file, String fileType, String validationKey) throws IOException {
        Logger.debug(TAG, "Type of file: " + fileType);
        Logger.info(TAG, "Uploading of file with name: " + file.getName());
        FileDataRequest wrappedFileData = WrapUtils.wrapUploadFileData(file); // any additional metadata
        MultipartBody.Part rawFile = WrapUtils.wrapFile(file, fileType); // wrap raw file into multipart body
        Call<UploadedFileResponse> uploadCall = mService.directUpload(wrappedFileData, rawFile, validationKey);

        Response<UploadedFileResponse> response = uploadCall.execute();
        // handling of response
    }
  
}

SomeRetrofitService.java

public interface SomeRetrofitService {
  
    @Multipart
    @POST("/upload?action=save")
    Call<UploadedFileResponse> directUpload(@Part("data") FileDataRequest data, @Part MultipartBody.Part rawFile, @Query("validationkey") String validationKey);
  
}

WrapUtils.java

public class WrapUtils {
  
  private static final String MEDIA_TYPE_MULTIPART_FORM_DATA = "multipart/form-data";
  
  public static MultipartBody.Part wrapFile(File file, String fileType) {
      RequestBody requestFile = new RequestBody(file, MediaType.parse(MEDIA_TYPE_MULTIPART_FORM_DATA));
      return MultipartBody.Part.createFormData(fileType, file.getName(), requestFile);
  }
  
}