From the idea of progrium I did a fun little hack last night.
(mimetype or file extension) -> icon == stdicon.com
In the vein of gravatar’s simple URLs, just add the file extension, or mimetype onto the path, and you will get a good icon representing it. There are more options for choosing your icon set, size, and default, but head over to the root page to find out more.
It’s open source, so fork and fix. And if you have any ideas for good icon sets, please post a comment or email me and I’ll get them into the system.