Kirk,
At least on the Windows version of PM, under the Launching tab of the Preferences dialog for "default application to edit photos" there are two options: (1) Let Windows open files, or (2) Assign default application. If "Let Windows open files" is selected, then any file type can easily be editted, even unsupported file types. However, when a default editor is set, it seems to be used for all files, not just photos. I think this option should only affect the editting of photo files. Other file types should probably always be opened by Windows. This would give us the ability to have both a default photo edittor AND easy ability to edit unsupported file types.
Thanks,
Ian