I receive a lot of emails with inline images (usually in the signature), when replying an email with inline images I've noticed that the MIME message generated by org-msg sets Content-Disposition as "attached" instead of "inline", this looks weird in clients like Outlook Web.
the only workaround I've found so far is to set
(setq mml-content-disposition-alist
'((text (rtf . "attachment") (t . "inline"))
(".*tmp.*cid.*image.*png" . "inline")
(t . "attachment"))))
this way mml forces content-disposition inline for attachments with paths matching the ".*tmp.*cid.*image.*png"" regex, which matches where org-msg stores that kind of attachments on my machine.
I'm sure there are lots of way this can break, but at least it I won't spam my colleagues with "fake" attachments.
I'm using the latest org-msg, mu4e 1.10.8 and Emacs 29.1