How do I show or hide file extensions in Windows 7?
(View instructions for Windows 8)
From the Start Menu, choose Control Panel.
Next, select Appearance and Personalization.
Click on Folder Options.
Lastly, uncheck the option for "Hide extensions for known file types" and click.
Windows 7 applies this option globally to all folders, so you will now see common file extensions such as ".txt" and ".jpg" displayed in each folder. If you want to hide file extensions in the future, you can simply recheck the same checkbox.