// The must headers.
    res.setHeader('Content-type', 'application/octet-stream');
    res.setHeader('Content-Disposition', 'attachment;filename=aaa.txt');    // 'aaa.txt' can be customized.
    var fileStream = fs.createReadStream('./download/aaa.txt');
    fileStream.on('data', function (data) {
        res.write(data, 'binary');
    });
    fileStream.on('end', function () {
        res.end();
    });