Currently we attach inline images like any other image. This work well as long as the cid is referenced in the email, most clients will hide the image from the attachment list. (https://github.com/thunderbird/appointment/blob/main/backend/src/appointment/controller/mailer.py#L116)

However it seems k9mail/tfa looks for Content-Disposition: inline which seems good and correct. So we should probably mark inline images as such.