Those file formats appear in the file association list because it works like a kind of filter only showing interesting files (btw There's now a "Show all files" option which I forgot to mention) but support (thumbnails, preview) is sometimes more problematic.
I tend not to add file formats P3dO doesn't support unless they can't be ignored (like max). But there's a minimal support for Bryce.

Yarp - author of P3DO Organizer for Poser