Actually, AFAIK (i'm a P4 user) .bum files are only handled by P5 by setting a special option, which disables most other materials options at the same time (essentially it resort to using the standard P4 rendering engine). BTW - usually only the .jpg file is included, even in P4 files, to keep the file small, leaving to the end user to convert it to a .bum, which isn't that long nor hard to do.