Converts given RDFValue to an RDFURN of a file to be downloaded.

def CollectionItemToAff4Path(item, client_id=None):
  """Converts given RDFValue to an RDFURN of a file to be downloaded."""
  if isinstance(item, rdf_flows.GrrMessage):
    client_id = item.source
    item = item.payload
  elif isinstance(item, rdf_flow_objects.FlowResult):
    client_id = item.client_id
    item = item.payload

  if not client_id:
    raise ValueError("Could not determine client_id.")
  elif isinstance(client_id, string_types):
    client_id = rdf_client.ClientURN(client_id)

  if isinstance(item, rdf_client_fs.StatEntry):
    return item.AFF4Path(client_id)
  elif isinstance(item, rdf_file_finder.FileFinderResult):
    return item.stat_entry.AFF4Path(client_id)
  elif isinstance(item, collectors.ArtifactFilesDownloaderResult):
    if item.HasField("downloaded_file"):
      return item.downloaded_file.AFF4Path(client_id)

  raise ItemNotExportableError(item)