Oh. I think you solved a different problem. What happens if I turn those associations off or on? (Mine seem to be all on by default.)
Then PM won't check for them for existence. Speeds up rescan a bit if you know that you won't be ever encountering a particular file extension (i.e. if you don't use Bibble, then there is no need to check for .bib files.)
If the files do exist and you have directed PM to ignore them (made them inactive) and you have proxies turned on in your View menu then they'll appear as proxies. Otherwise they won't appear at all.
We keep them all on by default because it eliminates support calls. You're free to deactivate any that you know you will never use.
-Kirk