[FIX] mail: create attachments on the correct record when using the popup editor

Send a message using the popup editor.
Drag and drop an image into the message content.
It will create an attachment for the inlined image.
However it did give a res_model value of mail.message but no res_id;
add_missing_default_value would fill in the res_id from the _context, which is
not the mail_message but the current object on which it applies
(e.g. helpdesk.ticket).
We put the correct res_model along with the res_id.

opw 1903194

closes odoo/odoo#28526

addons/mail/models/mail_message.py

@@ -767,7 +767,8 @@ class Message(models.Model):
                         'name': name,
                         'datas': match.group(2),
                         'datas_fname': name,
-                        'res_model': 'mail.message',
+                        'res_model': values.get('model'),
+                        'res_id': values.get('res_id'),
                     })
                     attachment.generate_access_token()
                     values['attachment_ids'].append((4, attachment.id))