More: Actually, if you accomplished your purpose I think your extensions should be showing, because it's default, but I don't know if you can toggle them off, because my experience has been that toggling that Finder pref only affects newly created items.
I toggle Finder > Preferences > Advanced > Show all filename extensions on and off regularly and it effects every file
regardless of when they were created*. The extensions are, always there, the only difference is whether or not Finder displays them.
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*Prior to OS X Apple used hidden parameter files instead of extensions to designate file type and what app they belonged to and if your files are from that era they may not have an extension. Such files would, of course, predate Numbers.
