Message preview with "Content-Transfer-Encoding: base64" wrong?

[SystemKeeper]

Steps to reproduce

The mail has set

Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64

and does render correctly when opening, just the preview shows the base64 encoded data?

Expected behavior

The preview should take base64 encoded data into account and show the actual contents.

Actual behavior

Base64 encoded data in preview.

Mail app version

5.7.0-beta.1

Nextcloud version

32.0.3

Mailserver or service

Exchange

Operating system

Debian 13

PHP engine version

Other

Nextcloud memory caching

APCu & Redis

Web server

Apache (supported)

Database

MariaDB

Additional info

PHP 8.4 is used. This is the first time the mailbox was connected to mail, so no idea if that is a regression.

[DerDreschner]

I tried to replicate the issue with several base64 encoded messages and was unable to do so. Even when the Content-Type is exactly the same as in your example.

Maybe you can send me the affected message or at least the headers over Talk, so that I can understand further why this happens there?

[DerDreschner]

The message itself is fine. It only happens when using Exchange as mail server. Were able to reproduce.

[DerDreschner]

Looks like the issue is the difference response to the following IMAP command sent by this code line:
C: 7 UID FETCH 1 (BODY.PEEK[1] BODY.PEEK[1.MIME] BODY[HEADER])

Dovecot responses with three seperate answers:
S: * 1 FETCH (UID 2 BODY[1] {252}
S: BODY[1.MIME] {1448}
S: BODY[HEADER] {1448}

while Exchange tries to be smart and combine the BODY[1.MIME] and BODY[HEADER] answer, as the content is the same:
S: * 1 FETCH (BODY[1] {252}
S: BODY[1.MIME] NIL BODY[HEADER] {1377}

Additionally, Exchange doesn't deliver the following two header lines that are being included in the original message (yet the size difference between Dovecot and Exchange):

X-MS-Exchange-Organization-AVStamp-Enterprise: 1.0
MIME-Version: 1.0

[SystemKeeper]

Thanks for digging into this!

I am not sure Exchange considers the content the same. When I only request the mime part (tried it manually, not within mail), nothing is returned:

D2 UID FETCH 356899 (BODY.PEEK[1.MIME])
* 72340 FETCH (BODY[1.MIME] NIL UID 356899)
D2 OK FETCH completed.

If it would consider it the same, I would expect it to return the headers? Does BODY[1.MIME] NIL indicate there's none?

The issue originally occurred on Exchange. While trying to figure out what's happening, I imported the mail to Stalwart and observed the same behavior. The response in this case is

C: 5 UID FETCH 28 (BODY.PEEK[1] BODY.PEEK[1.MIME])
S: * 1 FETCH (UID 28 BODY[1] {252}
S: [LITERAL DATA: 252 bytes]
S: )
S: 5 OK UID FETCH completed

Not sure if this is relevant or not...