I am using the following simplified code to make a multipart request with dio

  static Future<void> updateAvatar(http.MultipartFile file, Function(String) responseHandler) async {
    dio.FormData files = dio.FormData.fromMap({
      'file': file,
    });
    await HttpUtils.uploadWithDefaultClient<String?>('/service/user/current/avatar/update', files, null, (data){
      if (data == null){
        return;
      }
      String avatar = data;
      Global.user!.avatar = avatar;
      for (Function listener in avatarListeners) {
        listener(avatar);
      }
      responseHandler(avatar);
    });
  }

Future<custom.Response<Map<String, dynamic>?>> postMultipart(String url, FormData formData, [Function(int, int)? onProgress]) async {
    var dio = Dio();
    if (host != null){
      dio.options.baseUrl = host!;
    }
    dio.options.headers = {
      ...headers,
      'Cookie': 'worldstreet_at=$accessToken',
    };
    return await _handleDioRequest(dio.post(url, data: formData, onSendProgress: onProgress));
  }

However, dio keeps giving me http status code of 400. I believe there is something wrong with the request.

This is my server code using Spring Java:

@RequestMapping("/current/avatar/update")
public String updateCurrentUserAvatar(@RequestPart("file") MultipartFile file){
    Long uid = LoginUtils.getUid();
    User user = userService.selectByUid(uid);
    FileInfo fileInfo = fileService.uploadAvatar(file);
    ExceptionUtils.failIf(fileInfo == null, "upload failed");
    String avatar = fileInfo.getUrl();
    if (avatar != null){
        fileService.deleteFile(user.getAvatar());
        user.setAvatar(avatar);
    }
    userService.update(user);
    return avatar;
}

I am able to make the upload request using c# and postman, but flutter dio keeps telling me wrong.

I have tried changing server’s @RequestPart to @RequestParam, setting ContentType to `multipart/form-data` None just still not works.

Either changing the server or the frontend code is fine for me.

Thanks in advance!