docs: def upload_image(self, media_file):
          """
          上传群发消息内的图片
          详情请参考
          http://mp.weixin.qq.com/wiki/15/5380a4e6f02f2ffdc7981a8ed7a40753.html
  
          :param media_file: 要上传的文件，一个 File-object
          :return: 上传成功时返回图片 URL
          """
          res = self._post(
              url='media/uploadimg',
              files={
                  'media': media_file
              },
              result_processor=lambda x: x['url']
          )
          return res