On your OS X Mountain Lion computer, you want the Finder to display filename extensions...

Choose Preferences from the Finder menu, click Advanced, and select the option "Show all filename extensions."

Control-click anywhere on the Desktop, select Preferences from the pop-up menu, click Advanced, and select the option "Show all filename extensions."

Choose Show View Options from the View menu, and select the option "Show all filename extensions."

Select any file icon, and choose Get Info from the File menu. In the Info window, click the disclosure triangle next to Name & Extension, deselect the option "Hide extension," and click Change All.