Comment 3 for bug 533009

Well, bzr_hookless uses bzrlib's msg.add_inline_attachment function, and that has an optional mime_subtype argument, so the
necessary change would seem to be a one liner:

--- bzr_hookless_email.py 2012-03-22 15:15:15 +0000
+++ bzr_hookless_email.py 2012-09-05 07:37:15 +0000
@@ -287,7 +287,7 @@
                     diff_val += (" %d bytes (byte limit is %d)\n" %
                                  (numbytes, options.byte_limit))

- msg.add_inline_attachment(diff_val, diff_name)
+ msg.add_inline_attachment(diff_val, diff_name, 'x-diff')

         return msg