Disable file extension warning in OS X
Turn off warning when changing file extensions on your Mac
Here's how you can disable the annoying warning that appears when you go to change a file extension in OS X on your Mac.
1. Open a terminal window.
2. Type in the following command then hit Enter.
defaults write com.apple.finder FXEnableExtensionChangeWarning -bool false
For more, see the original article at the link below.