MacOS – How to force OSX to always save file extension

This SuperUser post worked for me:

You need to browse to the application that you’d like to use to open these files, and edit its bundle. Right-click the application to handle these file extensions and select Show Package Contents. Navigate to Contents, and edit Info.plist. You might need Property List Editor, which is part of Apple’s developer tools. If you’re lucky, it’s in XML format. Edit this file’s CFBundleDocumentTypes and add an entry for the extension that you want.

I used TextEdit as the default application to open .bar files. The only thing I did differently was change the XML that went in TextEdit’s Info.plist to:

    <dict>
        <key>CFBundleTypeExtensions</key>
        <array>
            <string>bar</string>
        </array>
        <key>CFBundleTypeIconFile</key>
        <string>BarDocument</string>
        <key>CFBundleTypeName</key>
        <string>Bar Document</string>
        <key>CFBundleTypeRole</key>
        <string>Document</string>
    </dict>

…and then I used LSRefresh.app to refresh TextEdit in the launch services database. Then it worked as you suggested, although it will probably break the code signature of Textedit (and any other signed application) on Lion.

1. Go to Finder
2. open Finder Preferences
3. Uncheck "Show warning before changing an extension"

Optionally, check "Show all filename extensions" to verify that Mac did not automatically append extensions when you add your own.