Does it work if you select multiple files (drag a rectangular selection box with the mouse, starting in a "blank" area of the file list in Finder, or hold down Cmd and click them all) then right-click, hold down Opt, and choose "Get Summary Info"? I'm not sure if that actually lets you change the show/hide extension option for multiple files, but if it does it's certainly easier than doing them individually. (Obviously, this is easier if all the files are in the same folder.)
Personally, I turn on all file extensions. In Finder, you can do this with the Finder | Preferences menu item, then choose the "Advanced" tab and check "Show all filename extensions" (it will take some getting used to seeing .app after all of your application bundles, but I really prefer this because it ensures I never accidentally hide an extension by forgetting to type one when I save the file or whatnot). I'm not saying this is the best option for you, but if you never want to hide file extensions, it's something you may wish to consider. Others may have to chime in if neither of these two options works for you.