Do not send avatar in unencrypted messages #7136
Description
There is a problem reported on the forum:
https://support.delta.chat/t/invalid-mime-content-single-text-value-size-32822-exceeded-allowed-maximum-32768-for-the-chat-user-avatar-header/4067
The easiest fix seems to be not to send avatars in unencrypted messages, even as "hidden" headers. We don't display such avatars anyway and show grey avatars instead for address-contacts, so sending them is not useful.