bug#34992: 26.1; message-send-mail-with-mailclient mangles MIME messages

[Eli Zaretskii]

> From: David Bremner <david@tethera.net>
> Date: Mon, 25 Mar 2019 09:43:18 -0300
> 
> 
> To verify this in emacs -q
> 
> (setq message-send-mail-function #'message-send-mail-with-mailclient)
> (message-mail)
> 
> now attach a file, and send.
> 
> Notice that the whole encoded mime message is wrapped in a text/plain
> part, which breaks most (all?) MUAs ability to read the message.
> 
> I'm not sure what a sensible fix is here.

One possibility would be for message-mode to refuse to add attachments
when message-send-mail-function is set up to use the system mail
client.  The user should be told to attach using the mail client
facilities instead.