def filename_for(self, subpath):
        """
        Returns the relative filename for the specified subpath, or the
        root filename if subpath is None.

        Raises werkzeug.exceptions.NotFound if the resulting path
        would fall out of the root directory.
        """
        try:
            filename = self.readme_for(subpath)
            return os.path.relpath(filename, self.root_directory)
        except ReadmeNotFoundError:
            return None