urb has it basically right.
Extensions are somewhat optional on a Mac depending on the program and what version a MacOS you are talking about, much like *nix.
What it sounds like is the resource fork of a Mac file. Mac files can have more then one fork. Since other fikle systems do not understand multi forked files it will end up as two files or just get discarded, either way the file ends up destroyed.
The PalmOS also has rsrc files so it may be a Palm file.