I have made in GIMP two .ico files for two file types that do not have any program or icon registered to them. they are .dls & .sf2, both of which are soundfont files. I have many programs that use these files. like SONY Acid using .dls. ModPlug Tracker also uses .dls. I have a midi converter that uses .sf2 and so does FL Studio. and Awave Studio uses all soundfont files.
I tried using this program as I was told to directly on the homepage of Google: http://www.howtogeek.com/howto/12383/change-a-file-types-icon-in-windows-7/ but neither .dls nor .sf2 files are listed anywhere in that program. Many other unknown file extensions are listed in there but not .dls or .sf2