[dart-dio] multi-part file should accept filename

Created by: ayushin

Right now multipart forms methods are generated with Uint8List type fields and hard-coded filename as the field name:

       Map<String, dynamic> formData = {};
               if (image != null) {
                   formData[r'image'] = MultipartFile.fromBytes(image, filename: r'image');
               }
       bodyData = FormData.fromMap(formData);

Which is incorrect, unsuitable for certain backends.

I think it is more suitable to generate these fields as MultipartFile and let the upstream handle the MultipartFile generation, just like dart generator does.