Images inserted in e-mails are not displayed in Thunderbird

[E-Concept-Applications]

Bug

Hello,

Images inserted in the body of a mail (for example, in the signature) are not displayed in Thundebird.

After some research, it seems that this is due to the Content-Type of the mail being set to "Multipart/Mixed" instead of "Multipart/Related".

I realized this when I switched from v16 to v18. In v16, images encoded in base64 directly in the HTML tag remained as they were. In v18 (or since v17, I don't know), they are now modified to be attached with a CID link.

Would it be possible to change the Content-Type of mails to "Multipart/Related" to ensure maximum compatibility for the display of sent mails?

Thank you!

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Environment Version

18.0.2

Environment OS

Ubuntu 20.04

Environment Web server

Apache 2.4.41

Environment PHP

8.1

Environment Database

MariaDB 10.3.38

Environment URL(s)

No response

Expected and actual behavior

No response

Steps to reproduce the behavior

No response

Attached files

No response