code: def normalize_extension(extension):
      
      extension = decode_path(extension)
      if extension is None:
          return
      if extension.startswith('.'):
          extension = extension[1:]
      if '.' in extension:
          _, extension = os.path.splitext(extension)
      extension = slugify(extension, sep='')
      if extension is None:
          return
      if len(extension):
          return extension
docs: Normalise a file name extension.