When you change a file extension in Mac OS, you are asked to confirm that you actually want to do so. This can be annoying to those who routinely change say, .txt extensions to .csv. Thanks to KirkMC’s post in MacWorld, a simple Terminal command will disable that confirmation popup:
defaults write com.apple.finder FXEnableExtensionChangeWarning -bool false
Then just relaunch Finder for the change to take effect:
killall Finder