使用dio库功能强大的Http客户端，支持拦截器，全局配置，表单数据，请求取消，文件下载，超时等。

FilePickerResult? result = await FilePicker.platform.pickFiles(
                      type: FileType.custom,
                      allowedExtensions: ['jpg', 'pdf', 'doc', 'png', 'mp4', 'mkv'],
                    );
                    if (result != null) {
                      PlatformFile file = result.files.first;
                
                      print(file.name);
                      print(file.bytes);
                      print(file.size);
                      print(file.extension);
                      print(file.path);
                 var multipartFile = await MultipartFile.fromFile(file.path,
                       );
                    FormData formData = FormData.fromMap({
                
                
                
                      "MediaFile": multipartFile,  //define your json data here
                    });
            
            
             functionToUploadToServer(formData); //call upload function passing multiform data
                }
    }
            
            
            
            
                functionToUploadToServer(multiformData){  
                    
                      Dio dio = Dio();
                
                      final response = await dio.post(
                        url,
                        data: multiformData,
                        options: Options(
                            contentType: 'multipart/form-data',
                            headers: {},
                            followRedirects: false,
                            validateStatus: (status) {
                              return status! <= 500;
                            }),
                      );
                      print(response.statusCode);
                     
                }

This _upload（）函数需要一个图像列表，带有标题的API url，然后将图像列表上传到服务器。

void _upload(List<XFile> files) async {
 for (var file in files) {
  String fname = filename;
  FormData data = FormData.fromMap({
   "file": await MultipartFile.fromFile(
     file.path,
     filename: fname,
    ),
  });

  Dio dio = new Dio();
 
  print('Image File Name: $fname');
  var response = await dio.put(url,
    data: data,
    options: Options(
      headers: {},
    ),
  );

  print(response.statusCode);
  print(response.data);
 }
}