def METHOD_NAME(client_id: str, flow_id: str) -> Optional[str]:
  """Retrieves a filesystem type information of the specified timeline flow.
  Args:
    client_id: An identifier of a client of the flow.
    flow_id: An identifier of the flow.
  Returns:
    A string representing filesystem type if available.
  """
  results = data_store.REL_DB.ReadFlowResults(
      client_id=client_id, flow_id=flow_id, offset=0, count=1)
  if not results:
    return None
  result = results[0].payload
  if not isinstance(result, rdf_timeline.TimelineResult):
    raise TypeError(f"Unexpected timeline result of type '{type(result)}'")
  return result.filesystem_type