[FIX] account: fix main_attachment_id not set on attachment upload

When creating invoices via the 'Upload' button, the
'message_main_attachment_id' field wasn't set on the record. This
happened because the message posted in the chatter was linked to the
wrong 'res_model' which prevented it from being selected as the main
attachment.

This was causing a bug in the OCR module ('account.invoice.extract')
where we use this field to determine which attachment must be sent to
the OCR server.

closes odoo/odoo#36309

Signed-off-by: Florian Daloze (fda) <fda@odoo.com>

addons/account/models/account.py

@@ -1044,7 +1044,7 @@ class AccountJournal(models.Model):
         invoices = self.env['account.move']
         for attachment in attachments:
             invoice = self.env['account.move'].create({})
-            attachment.write({'res_model': 'account.move', 'res_id': invoice.id})
+            attachment.write({'res_model': 'mail.compose.message'})
             invoice.message_post(attachment_ids=[attachment.id])
             invoices += invoice