code: def check_extension(extension: str, allow_multifile: bool = False):
      
      check_var(extension, var_types=str, var_name='extension')
  
      # Extension should either be 'multifile' or start with EXT_SEPARATOR and contain only one EXT_SEPARATOR
      if (extension.startswith(EXT_SEPARATOR) and extension.count(EXT_SEPARATOR) == 1) \
              or (allow_multifile and extension is MULTIFILE_EXT):
          # ok
          pass
      else:
          raise ValueError('\'extension\' should start with \'' + EXT_SEPARATOR + '\' and contain not other '
                           'occurrence of \'' + EXT_SEPARATOR + '\'' + (', or be equal to \'' + MULTIFILE_EXT + '\' (for '
                           'multifile object parsing)' if allow_multifile else ''))
docs: Utility method to check that the provided extension is valid. Extension should either be MULTIFILE_EXT
      (='multifile') or start with EXT_SEPARATOR (='.') and contain only one occurence of EXT_SEPARATOR
  
      :param extension:
      :param allow_multifile:
      :return: