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.