How to fix mail.template attachments by re-attaching them to some
template:

```sql
with template_attach as
 (select *
    from email_template_attachment_rel
   where attachment_id in (
          select id
            from ir_attachment
           where res_id = 0 and not public and res_model='mail.template')
 )
update ir_attachment a set res_id = ta.email_template_id
                      from template_attach ta
                     where a.id = ta.attachment_id;
```