code: def prepare_filename_decorator(fn):
      
      @wraps(fn)
      def inner(self, normalized_url, request):
          ext = settings.ROUGHPAGES_TEMPLATE_FILE_EXT
          if not normalized_url:
              normalized_url = settings.ROUGHPAGES_INDEX_FILENAME
          filenames = fn(self, normalized_url, request)
          filenames = [x + ext for x in filenames if x]
          return filenames
      return inner
docs: A decorator of `prepare_filename` method
  
      1. It automatically assign `settings.ROUGHPAGES_INDEX_FILENAME` if the
         `normalized_url` is ''.
      2. It automatically assign file extensions to the output list.