Which flavor of Windows are you running?
This is in the DDE in File associations, also known as a MIME type(I'll give EC to anyone who can tell me what MIME is, because I can't give you a word-for-word def. on MIME).
I can give you a broad description of how to diagnose it. Go into your explorer, then View-Folder options(may vary depending on OS), and then click on the extension. Then double-click on it, and go to OPEN. Double-click and paint a picture of what it shows.
If this doesn't make sense, then someone else can give you the non-Ozzy translation of what I just typed.