Does a File Extension Always Point to a Given Program?
Are there exceptions to the Extension = File Type under Windows?
With no official registry of file types there is absolutely no control over which program may claim which extension. Also, some programs allow you the freedom to name a file anything you want so long as you place them in a particular directory/folder.
While there is no "creator information" in Windows/DOS files ala the Macintosh, a few programs attempt to analyze the structure of a file and if that structure is recognized open the file. You can demonstrate this latter by an experiment with Microsoft Word (under Windows XP only)...
By this you can see that Microsoft is starting to build code similar to Apple's creator information into their operating systems. (Of course, for now it only seems to work with some Microsoft products.)