Fix sending multipart/related
This fixes writing the content-id for attachments in
MIME which was broken with
9f81ed6561c5f41e50d1a51333c9586a33ed2ef6
While this issue was hidden when we had related attachments
disabled due to T5982 it would also not work
in third party clients.
- Fixes-Commit: 9f81ed6561c5f41e50d1a51333c9586a33ed2ef6
- GnuPG-Bug-Id: T5982