[FIX] mail, mass_mailing: fix attachment ownership

Attachments that are uploaded on a record that isn't saved yet are
created with res_id set to 0. In the case of attachments linked through
a m2m rather than the usual (res_model, res_id), it means they may not
be readable afer the creation of the record, except by their creator.

Re-attaching them to the mailing / template at the end of the create()
call fixes the ownership.

Fixes #81935

closes odoo/odoo#82105

Signed-off-by: Olivier Dony <odo@odoo.com>