Not able to download file from MongoDB GridFS as stream with nodejs/expressjs

I am using NodeJs 1.9.x with express 3.x, mongoose 3.3.x and gridfs-stream modules. Following is upload code. Which is working great.

var mongoose = require('mongoose');
var fs       = require('fs');
var GridStrm = require('gridfs-stream');

exports.stream_ajax_upload_file = function(req, res){
    var fileType =  req.header('X-File-Type');
    var fileName =  req.header('X-File-Name');
    var uniqId   =  req.param('uniqId', '112211112111');
    var conn     = mongoose.createConnection('localhost', 'TestArun', 27017);
    conn.once('open', function () {

        var gfs     = GridStrm(conn.db, mongoose.mongo);
        var fileId  = new ObjectID(uniqId);
        writestream = gfs.createWriteStream(fileId, [{"content_type": fileType, "metadata":{ "fileName": fileName, "uploaded_at": (new Date()).toString() }}]);

        req.on('data', function(data) {
            console.log("Writing stream in to gridfs");
            writestream.write(data);
        });

        req.on('end', function(data) {
            writestream.end();
            res.send("I think it is uoploaded: ID::" + uniqId);
        });
    });

}

I am able to see the uploaded data in as chunks in gridfs. Following is the download code.

var mongoose = require('mongoose'); 
var fs       = require('fs');
var GridStrm = require('gridfs-stream');

exports.download_file_as_stream = function(req, res){
    var uniqId =  req.param('uniqId', '112211112111');
    var conn = mongoose.createConnection('localhost', 'TestArun', 27017);
    conn.once('open', function () {     
        var gfs         = GridStrm(conn.db, mongoose.mongo);
        var fileId      = new ObjectID(uniqId);
        var readstream  = gfs.createReadStream(fileId, [{"content_type": 'application/pdf'}]);
        //res.header('content-type','application/pdf');
        readstream.pipe(res);
    }); 
}

The issue is I am not able to download the proper file. When I try to download I am getting some file which is downloading in the name of request url i.e: download_file_as_stream. But the size is same as the uploaded file. I think the issue with content-type, etc setting. I am not sure where is the problem. Whether it is in upload or download section ?