.ICONSET File Extension
File TypeMac OS X Icon Set Folder
What is an ICONSET file?
If you have Xcode 4.4 or later, the Iconset Generator application can automatically create an ICONSET folder with 10 icons that are ready to be created as high-resolution ICNS files. You must supply a 1024x1024 PNG image for the ICONSET folder to be created.
Xcode can automatically create an ICNS file from your ICONSET folder. You must add the folder before you build the project into the application.
You can manually convert the contents of the ICONSET folder by running Terminal and entering the iconutil -c icns <iconset filename>. The <iconset filename> is the path to the location of your ICONSET folder.
NOTE: The folder must have the "iconset" extension in order for the folder to be recognized and the PNG files to be created as ICNS files.