code: def _filename(self_or_cls, filename):
          "Add the file extension if not already present"
          if not filename.endswith(self_or_cls.file_ext):
              return '%s.%s' % (filename, self_or_cls.file_ext)
          else:
              return filenamf _filename(self_or_cls, filename):
          "Add the file extension if not already present"
          if not filename.endswith(self_or_cls.file_ext):
              return '%s.%s' % (filename, self_or_cls.file_ext)
          else:
              return filename
docs: Add the file extension if not already present