def ext(self):
        """
        The extension portion of this pack URI, e.g. ``'xml'`` for
        ``'/word/document.xml'``. Note the period is not included.
        """
        # raw_ext is either empty string or starts with period, e.g. '.xml'
        raw_ext = posixpath.splitext(self)[1]
        return raw_ext[1:] if raw_ext.startswith('.') else raw_ext