[FIX] account_facturx: retrieve attachments in the right way to create an invoice

Before that, we relied on the fact the call to super() assigned the attachment_ids field, which was wrong outside mail.compose.message. Hence, we fix that by directly browsing the attachment.

closes odoo/odoo#36552

Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>