Nested MIME multipart/alternative block for mixed body type with attachments.

The mail library fails to properly distinguish file attachments from
the message body when there are both text and html versions present,
instead listing the attachments as another version of the body in
another alternative format. Where all three are present it is
necessary to explicitly define a separate MIME boundary which
distinguishes body versions from attachments.
Fixed encoding of html part.

HTML part was listed as quoted-printable but was not being encoded
as such, leading to a bug where any = symbol in the text would
cause the body to be truncated.