--<boundary>--

Without this change, some attachment (e.g. from gmail) would not be correctly
decoded because gmail would not insert any blank lines between the end of the
nested multipart/alternative part and the beginning of the attachment part:
--000000000000d75a0f058779bbb2--
--000000000000d75a12058779bbb4
Content-Type: image/jpeg;
	name="29571163_1640947089321419_3376478908098884084_n.jpg"
Content-Disposition: attachment;
	filename="29571163_1640947089321419_3376478908098884084_n.jpg"
Content-Transfer-Encoding: base64

It looks (from RFC2046) like boundary delimeters should actually be:
"\r\n--<boundary>", but I'll look into that later.