code: def post_file(self, url, filename, file_stream, *args, **kwargs):
          
          res = self._conn.post(url, files={filename: file_stream},
                                headers=self._prepare_headers(**kwargs))
          if res.status_code == 200 or res.status_code == 201:
              return res.text
          else:
              return None
docs: Uploads file to provided url.
  
          Returns contents as text
  
          Args:
              **url**: address where to upload file
  
              **filename**: Name of the uploaded file
  
              **file_stream**: file like object to upload
  
              .. versionadded:: 0.3.2
                  **additional_headers**: (optional) Additional headers
                  to be used with request
  
          Returns:
              string