Maybe how it should work is that the first time the user opens a .c file (or other related suffix) they get prompted for which mode they would like to handle it

Maybe how it should work is that the first time the user opens a .c file (or other related suffix) they get prompted for which mode they would like to handle it with. The choice is then remembered, but with the option not to remember ("just this once"). Gee, either I've seen that before or I'm a UX visionary.