Hi,

You can attach the binary field like this;

self.ensure_one()
        mail_template = self.env.ref(
            'airport_lounge.agency_reservation_export_email_template',
            raise_if_not_found=False)
 
        ctx = {
            'default_model': 'reservation.wizard',
            'default_use_template': bool(mail_template),
            'default_template_id': mail_template.id if mail_template else None,
            'default_composition_mode': 'comment',
            'mark_so_as_sent': True,
            'default_attachment_ids': self.attachment.ids,
            'default_email_layout_xmlid': 'mail.mail_notification_layout_with_responsible_signature',
            'force_email': True,
        }
        return {
            'name': _('Send Invoice'),
            'type': 'ir.actions.act_window',
            'view_mode': 'form',
            'res_model': 'mail.compose.message',
            'views': [(False, 'form')],
            'view_id': False,
            'target': 'new',
            'context': ctx,
        }     

Regards