(Thanks to Donald N on themailing list for the detailed description of how this works)
Mulberry uses the old Internet Config system to determine MIME-types on outgoing as well as incoming attachments. The only way to modify these settings in Mac OS X is by using MisFox (or possibly, if you can get it to launch, Internet Explorer). Unfortunately, since Mac OS X 10.6 (Snow Leopard), file type/creator are no longer used by the operating system when determining how to launch an application. In its place Apple added Launch Services. Read more about that at TidBITS and ars technica.