[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>