Support multipart messages

An ancient issue, but still relevant - also related to ✨ Render HTML message in iFrame Needs work .

Looking at https://github.com/plancake/official-library-php-email-parser, it is no longer maintained, and the readme links to https://stackoverflow.com/questions/4721410/best-way-to-handle-email-par...

One of the suggestions there is https://github.com/zbateson/mail-mime-parser - I'll give that a try.

Inmail → currently has no supported version, and this requirement is more generic, so changing the title.