To reproduce

• Compose a new message, attach a .eml file
• (encrypt to make sure the result is not affected by outlook)
• Decrypt/view the received message

Result/observations:
The attached message is rendered inline (questionable, but justifiable), and broken. Diff shows that line-breaks are different etc. The message will most likely fail to parse, and any signatures are broken.

The same does not happen when attaching the exact same file with .mbox file-ending. This is shown as attachment, and preserved, correctly.

Notably, in the former case, the composer shows the attachment type as message/rfc822, but in the latter as application/mbox.

Suspected cause:
Special handling of message/rfc in ComposerViewBase and several Attachment* classes.

Caveats
The special handling may have a reason in forwarding / replies. Make sure to check those usecases, carefully.