code: def _is_valid_extension(extension):
      
      if not cfg.CONF.valid_extensions:
          return True
  
      valid = False
      for cext in cfg.CONF.valid_extensions:
          if not cext.startswith('.'):
              cext = '.%s' % cext
          if extension == cext:
              valid = True
              break
  
      return valid
docs: Checks if the file extension is blacklisted in valid_extensions.
  
      :param str extension: a file extension to check
      :returns: flag indicating if the extension is valid based on list of
                valid extensions.
      :rtype: bool