By default in OS X, when you change an extension for a file, you get a warning. This is somewhat annoying to me, as I do this pretty frequently and have never almost accidentally done so. So to disable, send a FXEnable ExtensionChangeWarning key into com.apple.finder as false:
defaults write com.apple.finder FXEnableExtensionChangeWarning -bool false
To then undo, simply run with a true key:
defaults write com.apple.finder FXEnableExtensionChangeWarning -bool true
