That probaly because your email client has automagically converted it for you. If I look at the raw email I see;
Mea Culpa. It's unlikely to be Mozilla (Thunderbird), I guess some relay along the way is stripping digital signatures.
That's quite serious and I hadn't suspected such a thing would ever happen - if you send something signed it's pretty important that it gets through signed! Maybe I should go beat up IT ... apologies for jumping to conclusions :(