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#81935closesodoo/odoo#82120
X-original-commit: 05fb705f2729ffdf07ab36accda20415a44e1aa6
Signed-off-by: Olivier Dony <odo@odoo.com>