These tests fails for html-attachments2.mbox, as this is a
multipart/related message. And for multipart/related it is unclear for me, if
this is a expected outcome or not.

It can't be uses as a decryption oracle, as the related node parts are taken from
the original message without any modification, so they keep their encryption state.

See thunderbird bug about this: https://bugzilla.mozilla.org/show_bug.cgi?id=560210