The MIME type multipart/mixed is used for the mail messages used in mail.cgi, wio-lib.pl and acct.pl Clients such as Thunderbird, Outlook and Roundcube look at the Content-Type multipart/mixed and treat this as meaning that an attachment is provided even when there is no Disposition attachment in the email source. RFC1341 does not indicate that multipart/mixed means an attachment is attached, only that there are different sections containing different content types. Therefore the use of multipart/mixed in IPFire is not incorrect but in many email clients it results in messages being flagged as having attachments when they don't.
acct.pl sends a pdf attachment with its messages therefore this must stay with multipart/mixed. mail.cgi and wio-lib.pl only send a single text message with no attachments or separate sections of different content. These two usages, if changed to multipart/alternative stop Thunderbird or Roundcube from showing the attachment icon with those messages. I have tested this change out and confirmed that changing from multipart/mixed to multipart/alternativce for these two usages fixes the problem of the attachment icon being shown in Thunderbird or Roundcube.
Oh sorry for being too fast with assigning this to someone :)
Bug fix created and tested out. Changed multipart/mixed to multipart/alternative in mail.cgi and wio-lib.pl Patches submitted to dev mailing list and patchwork. https://lists.ipfire.org/pipermail/development/2023-February/015469.html https://lists.ipfire.org/pipermail/development/2023-February/015470.html https://patchwork.ipfire.org/project/ipfire/list/?series=3408
Thank you very much for these! https://git.ipfire.org/?p=ipfire-2.x.git;a=commit;h=670981e64870b43981617560ce96b41db3a462fb https://git.ipfire.org/?p=ipfire-2.x.git;a=commit;h=c151ca241bfea1ea84b92269cd824971aabea732
https://blog.ipfire.org/post/ipfire-2-27-core-update-174-is-available-for-testing
This has been tested on my vm testbed with CU174 Testing. Sending a test email from the Mail Service page in the WUI no longer results in an attachment icon being shown for the message. This confirms that the bug has been fixed.