Calls the Update() method of a given VFSFile/VFSDirectory object.

def Start(self):
    """Calls the Update() method of a given VFSFile/VFSDirectory object."""
    self.Init()
    fd = aff4.FACTORY.Open(self.args.vfs_file_urn, mode="rw", token=self.token)

    # Account for implicit directories.
    if fd.Get(fd.Schema.TYPE) is None:
      fd = fd.Upgrade(standard.VFSDirectory)

    self.state.get_file_flow_urn = fd.Update(attribute=self.args.attribute)