A file name extension is a set of characters added to the end of a file name that determine which program should open it. Follow these steps to choose whether Windows displays these file extensions.
Open Folder Options by clicking the Start button , clicking Control Panel, clicking Appearance and Personalization, and then clicking Folder Options.
Click the View tab, and then, under Advanced settings, do one of the following:
To hide file extensions, select the Hide extensions for known file types check box, and then click OK.
To display file extensions, clear the Hide extensions for known file types check box, and then click OK.