I hope it's not too much of a faux pas
to add to such an old topic, but since this came up as a top result in a Google search for info on editing my Windows 7 icons I decided to go ahead and do so.
I wanted to change the same "unknown file" icon type, and didn't feel comfortable mucking about in my registry so I did some searching and found a walkthrough and an application that worked very nicely for me!
Blog post: Using Default Programs Editor to Change File Type Icons in Windows 7 and Windows Vista
(Found via Microsoft Support
And also, here is a link to the CNet page
for the utility used.
For my icon, I recolored the mobi/azw icon to a lovely green close to that of the official epub logo. A nifty Photoshop plugin
came in handy in saving the .ico filetype.