Flutter Multipart Request with headers and other fields

I want to send a Multipart request to my server like following :

{
  "file" : some_file.jpg,
  "room" : {
             "roomName" : "Room Name"
           }
}

I have certain headers

{
  "authorisation" : "Basic header"
  "screenInfo" : "screenInfo",
  "operation" : "operation",
  "userId" : "integer",
}

I have tried http.Multipart request as well as Dio package, but I am unable to send the request properly

What I have Used :

1. http.MultipartRequest

   final length = _mWebRequest.getRequestFiles()["file"].lengthSync();
   print("Length : $length");
    var request = http.MultipartRequest('POST', Uri.parse(urlString))
      ..headers.addAll(_mWebRequest.getHeaders())
      ..fields.addAll(_mWebRequest.getRequestParam())
      ..files.add(
        http.MultipartFile.fromBytes('file',
            _mWebRequest.getRequestFiles()["file"].readAsBytesSync(),
            filename: _mWebRequest
                .getRequestFiles()["file"]
                .path
                .split("/")
                .last),
      );
   

2. Dio package

   Dio dio = new Dio();
    var formData = FormData.fromMap({
      "room": {"roomName": "Custom Room 1", "isDefault": "false"},
      "file": await MultipartFile.fromFile(
          _mWebRequest.getRequestFiles()["file"].path,
          filename:
              _mWebRequest.getRequestFiles()["file"].path.split("/").last)
    });
    dio.options.headers.addAll(_mWebRequest.getHeaders());
    var response = await dio.post(urlString, data: formData);
   

Can someone help me out ?