file_name() returns the name of the file from which <ob> was loaded.
If the object is a cloned object, then file_name() will not be an
actual file on disk, but will be the name of the file from which the
object was originally cloned, appended with an octothorpe (#) and the
object instance number. Object instance numbers start at 0 when the
game is booted, and increase by one for each object cloned, hence the
number is unique for each cloned object. <ob> defaults to
this_object() if not specified.