I'm afraid I did this the hard way (no easy filters to use).
If you double-click an .icns file it will open in Preview. You can then save each size as a .png to edit in photoshop (check /System/Library/CoreServices and show the package contents of CoreTypes.bundle for generic .icns)
I tried my best to match the shadow image color/opacity by eye, then I added shadows & highlights (you could do this with "bevel & emboss" but I wanted to fine-tune it).
The shadow image itself has a gradient, just like the folder.