For what it's worth, the mailer that I use (GNUS) seems to think that
text/x-patch is the appropriate MIME type for patches. Certainly I
agree that application/octet-stream is a bad idea, and frankly I think
just inserting patches into the buffer instead of attaching them is
easiest. But there's a case to be made for attaching patches instead;
if so, I think text/x-patch is the way to go. Hopefully, in that
case, any mailer will notice the text/ part and will be willing to
display the contents without too much coercion.