I thought that, if you wanted to set the icon for a non-application file in OS X, you had to go through a convoluted process that involved copying the icon from another document. But it turns out you can just compose your icon with your favorite graphics tools, then copy and paste it into place.
For example, I wanted to change the icon on a QuickTime movie file. Here's what I had to do:
- Draw the icon graphics in OmniGraffle.
- Select the icon graphics.
- Edit->Copy (Command-C).
- Switch to the Finder.
- Select the QuickTime movie file.
- File->Get Info (Command-I).
- Select the document icon in the top left of the info window.
- Paste in the new imagery (Command-V).