IsoDraw change default extension

Renaming in the method is easy, but doesn't address the problem. What we've found is that the 'Open with IsoDraw' function that is used extensively by our illustrators is not yet supported with .idr. Thus, I would like to force the extension back to .iso. I could do this by macro, but yet additional work because of what appears to be a half-done integration of a new file extension. When I posed the question to PTC I received a copy/paste from their documentation just stating that .idr is the new file extension and a sentence saying that it can not be done. Oh well.