You can turn a directory full of .PNG files into an .ICNS or vice-versa using iconutil. It comes with OS/X and is a command-line program:
https://developer.apple.com/library/mac/documentation/Darwin/Reference/Manpages/man1/iconutil.1.html
You can also use Preview to extract individual PNGs out of the file. But to extract all the icons from the file you mention with their appropriate labels -- start up a command terminal in the directory where you want your output, and run this command:
iconutil -c iconset /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/GenericDocumentIcon.icns -o ./GenericDocumentIcon.iconset
If you're going the reverse direction, note that when building an .ICNS the directory you are building it from must end in .iconset and the filenames are very specific. The files in the directory must be named:
- icon_16x16.png
- icon_16x16@2x.png
- icon_32x32.png
- icon_32x32@2x.png
- icon_128x128.png
- icon_128x128@2x.png
- icon_256x256.png
- icon_256x256@2x.png
- icon_512x512.png
- icon_512x512@2x.png
For details on that and what the 2x is about, see High Resolution Guidelines for OS/X
The default page curl is rather "oppressive", especially at the small sizes like 16x16. Perhaps it looks good on a blank document icon, but when you're putting an image on it gets in the way. Document icons I looked at up close at that resolution often soften the gray edge on the curl and generally lighten up the darkness.
For instance, Google Chrome:
vs. the default:
I point it out just because I think it's a bit of a lost cause to make decent looking document icons at 16x16 if you follow their page curl there verbatim.
