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#1
As far as I know, Windows only allows changing icons of shortcuts and folders. What you are trying to do is to change the icon of rx3 document files which Windows does not allow (or yes, maybe through complex registry editing).
You will need third party icon tweaking utility to do so. This: Download IconTweaker on CrystalXP.net - Explorer tools is a clean such tool (which I have already used) which may help you achieve want you want. This tool does not break the system although I strongly advise you to create a system restore point before you install and use it since I have never tried it on Windows 7.
See if this small program doesn't work for you
Change a File Types Icon in Windows 7
I agree that a System Restore point at the least is a good idea. A system image is even better
Backup Complete Computer - Create an Image Backup
A Guy