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Originally Posted by
GrumpyDiver
Helen is quite right - under Windows7 at least, when you open Explorer click on the <Tools> menu item and select <Folder Options...>. This will bring up a pop up with three tabs. Click on the <View> tab and you will see a long list of check boxes' the eighth one down reads "Hide extensions for known file types". Check that and click the <Okay> but and you will be back to the way you were.
XP and Vista are quite similar, but I'm not sure if they are identical.
I personally always leave the extensions visible, but that is because it makes more sense for me to do so.