One thing i've noticed with Mozilla (and Firebird) is that it seems to want to save application/octet-stream stuff with an extra suffix that it picks up seemingly at random; my copy of Firebird wants to save .exe files as .swf (so they end up as filename.exe.swf), and on the boy's computer, Mozilla wants to save them as .ace. I could file a bug report on this... but i expect they're already working on it.