Assuming you have file extensions shown in the OS X Finder, attempting to change a file extension causes a warning dialog with a confirmation box to appear. This is annoying if you know what you’re doing and you have a compelling reason to change extensions, so let’s turn it off using the command line and defaults write.
First, open the Terminal, located within the /Applications/Utilities/ directory, then copy and paste in the following command:
defaults write com.apple.finder FXEnableExtensionChangeWarning -bool false
Follow that up by relaunching the Finder with killall:
killall Finder
To reverse the change and get the warning back when you attempt to change file extensions, use the following defaults write command:
defaults write com.apple.finder FXEnableExtensionChangeWarning -bool true
Kill the Finder again for changes to take effect.
转自:http://osxdaily.com/2012/04/11/disable-the-file-extension-change-warning-in-mac-os-x/