A file name extension is a set of characters added to the end of a file name that determine which program should open it. For more information, see File names and file name extensions: frequently asked questions.
Windows hides file name extensions, but you can choose to make them visible.
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 show file name extensions, clear the Hide extensions for known file types check box, and then click OK.
To hide file name extensions, select the Hide extensions for known file types check box, and then click OK.