Notes: This currently relies on CryptoHelper::decryptMessage() in mimetreeparser. That in turn re-implements all the decrypt handling that we already have elsewhere, except it fails on a lot of the (many) corner cases. In Outlook, it already breaks at the message no longer being properly typed as multipart/encrypted.

What we should do, instead, is to rely on the already parsed representation, and reassemble that into MIME format. We don't even need a complete implementation (e.g. of multipart/alternative), as long as we warn users, when we may have dropped stuff. We're already dropping crypto information, anyway.