WordPress MU

On line 70 of wpmu-functions.php there is a filter calling a hook called fileupload_init:
add_filter('fileupload_init','wpmu_checkAvailableSpace');

The only thing is that I can't find the hook anywhere in the source code. Can anyone shed some light on this?

OK - I found that this was referenced in wp-admin/upload.php but this file was removed a while back. I think I've found something else I can use though.

Cool!! Thanks for the lead. :)