I use this on linux with AssoGiate, an editor of the MIME file types database for GNOME.
It allows users and administrators to customize the detection and presentation of file types by GNOME and other environments
so I have the icons I want in nautilus(gnome file manager) for *.xrnx *.xrni *.xrnt *.rns
if your on linux and you have done renoise install.sh
you can install AssoGiate, run it and search for renoise to replace the icons