code: def file_open(self, fn):
          
          yield b'--'
          yield self.boundary.encode()
          yield CRLF
          headers = content_disposition(fn)
          headers.update(content_type(fn))
          for c in self._write_headers(headers):
              yield c
docs: Yields the opening text of a file section in multipart HTTP.
  
          Parameters
          ----------
          fn : str
              Filename for the file being opened and added to the HTTP body