I think I\'m literally on the last step before submitting a new Mac app, and I\'ve been banging my head against a wall for the past few hours on what I think should be really, r
To my knowledge, Xcode doesn't currently support using .xcassets for document icons. This is a little strange—the .xcassets editor clearly allows you to create new icons, but it doesn't generate .icns files, it just puts all the files in the same folder and uses a plist to manage them.
To create an icon for your document types, upload the 1024x1024 version of the icon to http://iconverticons.com/online/ to convert it to a .icns file. Alternatively, you could use a third-party tool to generate an .icns file with different images for different resolutions. There used to be a great tool included in the Developer Tools for doing this, but I can't seem to find it anymore.
Once you've got an icns file, add it to your project and you should be able to select it as the "Icon" for your Document Type or External/Internal UTI. Enjoy!
To be honest, it might be worth filing a bug about this. Xcode's development seems so strongly driven by iOS these days, they may not have noticed this yet!