docs: def get_comic_format(filename):
      """Return the comic format if it is a comic archive."""
      image_format = None
      filename_ext = os.path.splitext(filename)[-1].lower()
      if filename_ext in _COMIC_EXTS:
          if zipfile.is_zipfile(filename):
              image_format = _CBZ_FORMAT
          elif rarfile.is_rarfile(filename):
              image_format = _CBR_FORMAT
      return image_format