Take care of fixing validation of "inline"-attachments in mail-gpg.
We don't want to implement outgoing "inline"-formatting, but we need to be able to reliably read it.
See https://git.codecoop.org/schleuder/schleuder3/issues/11#note_958
Just tested it and schleuder successfully decrypts and verifies a single inline ciphertext. An inline ciphertext with an encrypted+signed attachment is reported as encrypted but not signed, though.