Doug,
Here's one such utility:
http://www.shelllabs.com/icon_changer.htm
The utility adds a "change icon" item to the context menu and has a search for icons feature.
You can also change the icon using the builtin features of Windows with just a bit more work.
Start Windows Explorer and click on the menu item "Tools > Folder Options..."
Select the "File Types" tab and scroll down the registered file types list until you see the suffix you want. Then, click on the suffix line to select it. Now click on the "advanced" button at the bottom of the dialog.
An "Edit File Type" dialog box will appear. At the top,there is a "Change Icon..." button, click it and you will get the "Change Icon" dialog where you can either select a new icon from the ones in the preselected file or you can browse to another file to pull its icons. You would need to know where that Mickey Mouse icon file is located though.