[FIX] mail_template: correctly compute / update attachment_ids in send mail

Attachments when generating an email based on a template are done in two
steps, see notably [1]. However mail generation replaces attachments using
a 6 command. This means existing attachments given through email_values
are not correctly managed and this leads to invalid command given to the
m2m field.

This commit fixes that by correctly generating a list of commands, using
4 (add) for new attachments coming notably from reports linked to the template.

Related task ID 1965040
PR #32459

[1] https://github.com/odoo/odoo/commit/f789005c9a6c26322565a1af7cc4e57a60eab9f0

addons/mail/models/mail_template.py

@@ -512,6 +512,7 @@ class MailTemplate(models.Model):
         # create a mail_mail based on values, without attachments
         values = self.generate_email(res_id)
         values['recipient_ids'] = [(4, pid) for pid in values.get('partner_ids', list())]
+        values['attachment_ids'] = [(4, aid) for aid in values.get('attachment_ids', list())]
         values.update(email_values or {})
         attachment_ids = values.pop('attachment_ids', [])
         attachments = values.pop('attachments', [])
@@ -545,10 +546,9 @@ class MailTemplate(models.Model):
                 'res_model': 'mail.message',
                 'res_id': mail.mail_message_id.id,
             }
-            attachment_ids.append(Attachment.create(attachment_data).id)
+            attachment_ids.append((4, Attachment.create(attachment_data).id))
         if attachment_ids:
-            values['attachment_ids'] = [(6, 0, attachment_ids)]
-            mail.write({'attachment_ids': [(6, 0, attachment_ids)]})
+            mail.write({'attachment_ids': attachment_ids})
 
         if force_send:
             mail.send(raise_exception=raise_exception)