Add TNEF support

Implement TNEF (Transport Neutral Encapsulation Format) support in Mailspring.

Partially implemented in Thunderbird via Lookout addon(s)

(Originally posted by salaros on GitHub.)

Ahh this is an interesting one—it looks like we can essentially “unpack” the .dat file and it gives us the real file within it? If we can find a tool on that page that has an open / LGPL license we could probably bundle it or it’s code with Mailspring.

(Originally posted by bengotow on GitHub.)

Apparently this would allow us to work with winmail.dat.

There are a couple of NodeJS projects for this:

So apparently there open-source tools, Mailspring not supporting TNEF is one of the reasons why I cannot migrate to it, because many people I receive emails from use Outlook, which sends attachment in TNEF format from time to time

This would make the most sense to me as a plugin, which could later be added to the core plugins.