Mail attachment does not work / header problems

Steps to reproduce

Open mail app (web interface)
Go to "Neue Nachricht" (new message)
Select a recipient, write subject and message body
Go to "..." at the bottom of the mail interface, then select "Anhang hochladen" (upload attachment)
Select an attachment from the File Selection Box
Press "SEND"
Look at the mail that was sent

I am using Nextcloud Hub 8 29.0.5 with Mail 3.7.7, but this also happened with older nextcloud versions 28.* and 29.0.x with corresponding older mail clients. Language is German.

Expected behavior

After step 7, I should see a mail with the attachment in another mail client (or in the mail app, if I sent a mail to myself)

Actual behavior

I see a mail, but the attachment is not there. I see a text "This message is in MIME format"

Mail app version

3.7.7

Mailserver or service

exim as mailserver; happens with SMPT transpart, as well as with sendmail interface

Operating system

Debian Linux

PHP engine version

PHP 8.2

Web server

Apache (supported)

Database

MariaDB

Additional info

Looking at the mail in maildiir format, I can see that the headers seem not to be formatted correctly:

From: XXXredactedXXX <XXXredactedXXX@XXXredactedXXX> Cc: Message-ID: <20240828123116.Horde.GesoMkoURhU5Gojp60h8exU@XXXredactedXXX> User-Agent: Horde Application Framework 5 Date: Wed, 28 Aug 2024 12:31:16 +0000 Content-Type: multipart/mixed; boundary="=_gPXHtDyO8TUAmLNl8o7IgH_" MIME-Version: 1.0 Message-Id: <E1sjHpU-00DBWN-1B@XXXredactedXXX>

Please note the space as first char in the lines Cc:, Message-ID: (the first one), User-Agent:, Date:, Content-Type: and MIME-Version:. Somehow, it seems the lines are not properly ended (perhaps, some CR/LF vs LF vs CR issue?)

That's also why the MTA creates a new Message-ID for this mail. This explains why the MIME data is not processed, because the headers are missing.

nextcloud / mail