Set content-type to multipart/mixed explicitly
In the present version, the notes with attachments (e.g. images in KJots) are broken: the mime-type of the whole message is "text/html" so attachments are interpreted as a part of the note.
I suspect that it was broken by commit bc800657.
This patch explicitly sets mime-type to "multipart/mixed" and adds the body of the note as the first part (with proper content-type on its own). Attachments go after that as subsequent parts.
Note: now all notes (even without attachments) will be "multipart/mixed",
but just with a single part. But it seems like it doesn't break anything
(reading code uses mainBodyPart
which automatically picks the first
part)