code: function streamCallback(data) {
  	var strFilename = "Node-Demo-Streamed-Callback.pptx";
  
  	app.get('/', (req,res) => {
  		res.writeHead(200, { 'Content-disposition':'attachment;filename='+strFilename, 'Content-Length':data.length });
  		res.end(new Buffer(data, 'binary'));
  	});
  
  	app.listen(3000, () => {
  		console.log('PptxGenJS Node Demo app listening on port 3000!');
  		console.log('Visit: http://localhost:3000/');
  		console.log('(press Ctrl-C to quit app)');
  	});
  }
docs: EX: Callback that receives the PPT binary data - use this to stream file