http://bugs.winehq.org/show_bug.cgi?id=19225
--- Comment #5 from Juan Lang juan_lang@yahoo.com 2009-07-07 19:01:15 --- (In reply to comment #4)
Actually, looks like I misled you (sorry!). Seems XP does append .txt if the filename doesn't have an extension. If it does have a non .txt extension, XP preserves it (wine does the same).
Does the behavior on Windows depend on the "hide extension" Explorer setting? I forget its exact name, but it's the one that hides file extensions for known file types, and which is set by default on Windows.