Oh and one more thing - even if you use the extension filter you should test the extension of hte file they've chosen. Why? Well, even if your filter is set up only for .MAP files, a user can still manually enter the name of a file or do something like type *.* in the file selector and get access to all of the files. Really, the filter is there as a convenience to the end user and although it's unlikely that they'd go through the trouble to select an invalid file with the filter in place, it's still not good programming practice to rely on getting a good result.
So use the filter so the end user can more easily find the files they want, but still test the extension of the file you're getting (or test the file itself for the correct kind of header - if your map files have headers -)
So use the filter so the end user can more easily find the files they want, but still test the extension of the file you're getting (or test the file itself for the correct kind of header - if your map files have headers -)