Displaying File extensions
By default, Windows hides the 3 or 4-letter file extensions for known file types and assigns them automatically when you save files from within an application. To change the default setting to display file extensions for all files, click Start, and then click computer. On the Command bar, click the Organize button, click Folder and Search Options, and then click the View tab. Under Advanced Settings, clear the Hide extensions for known file types check box, and then click OK.