I came across this when trying to umount a cdrom that was mounted with xffm. If a file that has no mime mapping is accessed or even just doing a right-click to view the pull-down menu, xffm will never release the file handle for that file. I used lsof to verify this. The only way to unmount the cdrom was to quit xffm and start a new one. Opening and closing files that has a known mime handler will not cause this problem.
Ah! So that's what it is! I could never figure this one out. Thank you much for this valuable information. :-) It is now fixed in CVS. The problem was a file descriptor left open by libxfcegui4/xfce4-modules.