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

def file_open(self, fn):
        """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
        """
        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