<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def prepare_filename_decorator(fn):
 """ 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. """

    @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