[FIX] mail: set message_main_attachment_id when the attachment is added via widget

Create a record.
Add an attachment, using the widget (aptly named 'add an attachment').
It is not set as message_main_attachment_id.
If you add the message through 'log note', then it is.

We add a hook to make sure that it is set as message_main_attachment_id
when added through the widget.

opw 1950403

closes odoo/odoo#31847

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>