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. The warning text says “Are you sure you want to change the extension from (this) to (that)?” then giving you two choices; keep the current file extension, or use the new extension.

That dialog box can be annoying if you know what you’re doing and you have a compelling reason to change extensions, which is often the case for advanced Mac users, so let’s turn it off using one of two methods in OS X; with the Finder Prefs panel, or the command line and defaults write.

How to Turn Off File Extension Change Warning in Mac OS X

The easiest way to stop the file extension change warning is to disable it through Finder preferences, here’s how:

From the Finder, pull down the ‘Finder’ menu and chooes “Preferences” Go to the “Advanced” tab Uncheck the box for “Show warning before changing an extension” Close preferences to set the change

Of course many users prefer to use the Terminal for a variety of reasons, and there’s a way to make the change through a defaults command string as well. This is great for remote management and automation.

Disable File Extension Change Warning with defaults

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.