code: def ext(self):
          
          # 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
docs: The extension portion of this pack URI, e.g. ``'xml'`` for
          ``'/word/document.xml'``. Note the period is not included.