Show or hide file name extensions
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.
To show or hide file name extensions
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Open Folder Options by clicking the Start button 
, clicking Control Panel, clicking Appearance and Personalization, and then clicking Folder Options.
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Click the View tab, and then, under Advanced settings, do one of the following:
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To show file name extensions, clear the Hide extensions for known file types check box, and then click OK.
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To hide file name extensions, select the Hide extensions for known file types check box, and then click OK.
