NAME
    file_name - find the filename of an object

    SYNOPSIS
    string file_name(object ob)

    DESCRIPTION
    This function gives the filename of an object concatenated with a possible instance number. This typically is a string on the form "/std/weapon#423". Instance numbers are unique for all objects in the game.

    If a filename has no instance number it is the 'master object' and not a 'cloned copy'.