How to send javascript buffer as a file from node.js server

I have html webpage with javascript script that uses XMLHttpRequest to get a file from a server. I want a browser to open a standard "download file" window, so a user can save a file. The code for that is simple:

var xhr;
xhr=new XMLHttpRequest();

xhr.onreadystatechange=function()
  {
  if (xhr.readyState==4 && xhr.status==200)
    {
    alert("File successfully received");
    }
  } 
xhr.open("GET”,”http://localhost:17001/filedl?param=1",true);
xhr.send();

On a server, I have a running node.js app with express module. The app is generating a buffer data and sending it back to the client as a file. The code is here:

var express = require('express');
var app = express();

app.get('/filedl', function(req, res){
  var pngBuffer = new Buffer("generated png goes here");
  res.setHeader('Content-Description','File Transfer');
  res.setHeader('Content-Disposition', 'attachment; filename=print.png');
  res.setHeader('Content-Type', 'application/octet-stream');
  res.end(pngBuffer);
});
app.listen(17001);

However, I can't get a "Save As" dialog open up in a browser. I'm just getting a response from my server with a buffer in a text form.

I've tried setting different response http headers, but without any luck.