Send attachment with completed order email

/**
 * https://media-x.hr/snippets/slanje-uvjeta-koristenja-u-mailu-zavrsena-narudzba/
 */
add_filter( 'woocommerce_email_attachments', 'mx_woocommerce_attachments', 10, 3 );


function mx_woocommerce_attachments($attachments, $email_id, $email_object){

    if( $email_id === 'customer_completed_order'){

        $attachments[] = get_attached_file(123);

    }
    return $attachments;
}