Method 'message_notify' is a simplified version of 'message_post' to use
to notify people of something, without posting on a document. It creates
a "user notification", a specific kind of message.
In this commit we make 'message_notify' process attachments like posting
does. That way the API is more similar between those two methods. Moreover
it allows to give 'attachments' to notify. Those are then transformed into
real 'attachment_ids' and converted if they come from the composer.
That way the mail composer can be used in notification mode with attachments.
This happens notably when using a template on a record that does not inherit
from MailThread. It is a classic usage of 'message_notify' which allows
notifying on documents that have no chatter, notably technical or admin
oriented models.
Followup of odoo/odoo#99482
Task-2710804 (Mail: Clean MailThread API)
Part-of: odoo/odoo#107356
Thibault Delavallée
e2c659eb0c