October 30, 2012, 3:39 PM — Here's how you can hide or unhide file extensions via the Get Info menu for a file in OS X Mountain Lion.
1. Select the file.
2. Press Command + I on your keyboard (or select File then Get info from the file's menu in the menubar).
3. Find the Name & Extension tab.
4. Click or unclick the "Hide extension" checkbox to see or hide the extension for that file.
Use the Get Info window in Mac OS X | Daily Mac Tips