Express helper, res.download
Express has a helper for this:
app.get('/xxx', function(req, res){
var file = __dirname + '/xxx';
res.download(file); // Set disposition and send it.
});
http file download header
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Disposition
res.setHeader('Content-disposition', 'attachment; filename=xxx');