docs: def walk_file(filename, walk_after, recurse=None, archive_mtime=None):
      """Optimize an individual file."""
      filename = os.path.normpath(filename)
  
      result_set = set()
  
      if _is_skippable(filename):
          return result_set
  
      walk_after = timestamp.get_walk_after(filename, walk_after)
  
      # File is a directory
      if os.path.isdir(filename):
          return walk_dir(filename, walk_after, recurse, archive_mtime)
  
      if _is_older_than_timestamp(filename, walk_after, archive_mtime):
          return result_set
  
      # Check image format
      try:
          image_format = detect_format.detect_file(filename)
      except Exception:
          res = Settings.pool.apply_async(stats.ReportStats,
                                          (filename,),
                                          {'error': "Detect Format"})
          result_set.add(res)
          image_format = False
  
      if not image_format:
          return result_set
  
      if Settings.list_only:
          # list only
          print("{}: {}".format(filename, image_format))
          return result_set
  
      if detect_format.is_format_selected(image_format, comic.FORMATS,
                                          comic.PROGRAMS):
          # comic archive
          result = walk_comic_archive(filename, image_format, walk_after)
      else:
          # regular image
          args = [filename, image_format, Settings]
          result = Settings.pool.apply_async(optimize.optimize_image,
                                             args=(args,))
      result_set.add(result)
      return result_set