无法使用nodeJS从MongoDB gridFS下载文件

Question

按照以下教程，我使用nodeJS和expressJS上传文件并将文件下载到gridFS。

https://ciphertrick.com/2017/02/28/file-upload-with-nodejs-and-gridfs-mongodb/

文件上传工作正常。但是当我尝试下载时，我只能在浏览器上看到该文件的内容（例如图像/文本）。但是我无法下载物理文件。任何帮助都感激不尽。谢谢！

    var express = require('express'); 
    var app = express(); 
    var bodyParser = require('body-parser');
    var mongoose = require('mongoose');
    mongoose.connect('mongodb://127.0.0.1/gridfstest');
    var conn = mongoose.connection;
    var multer = require('multer');
    var GridFsStorage = require('multer-gridfs-storage');
    var Grid = require('gridfs-stream');
    Grid.mongo = mongoose.mongo;
    var gfs = Grid(conn.db);

/** Seting up server to accept cross-origin browser requests */
app.use(function(req, res, next) { //allow cross origin requests
    res.setHeader("Access-Control-Allow-Methods", "POST, PUT, OPTIONS, DELETE, GET");
    res.header("Access-Control-Allow-Origin", "http://localhost:3000");
    res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
    res.header("Access-Control-Allow-Credentials", true);
    next();
});

app.use(bodyParser.json());

/** Setting up storage using multer-gridfs-storage */
var storage = GridFsStorage({
    gfs : gfs,
    filename: function (req, file, cb) {
        var datetimestamp = Date.now();
        cb(null, file.fieldname + '-' + datetimestamp + '.' + file.originalname.split('.')[file.originalname.split('.').length -1]);
    },
    /** With gridfs we can store aditional meta-data along with the file */
    metadata: function(req, file, cb) {
        cb(null, { originalname: file.originalname });
    },
    root: 'ctFiles' //root name for collection to store files into
});

var upload = multer({ //multer settings for single upload
    storage: storage
}).single('file');

/** API path that will upload the files */
app.post('/upload', function(req, res) {
    upload(req,res,function(err){
        if(err){
             res.json({error_code:1,err_desc:err});
             return;
        }
         res.json({error_code:0,err_desc:null});
    });
});

app.get('/file/:filename', function(req, res){
    gfs.collection('ctFiles'); //set collection name to lookup into

    /** First check if file exists */
    gfs.files.find({filename: req.params.filename}).toArray(function(err, files){
        if(!files || files.length === 0){
            return res.status(404).json({
                responseCode: 1,
                responseMessage: "error"
            });
        }
        /** create read stream */
        var readstream = gfs.createReadStream({
            filename: files[0].filename,
            root: "ctFiles"
        });
        /** set the proper content type */
        res.set('Content-Type', files[0].contentType)
        /** return response */
        return readstream.pipe(res);
    });
});

app.listen('3002', function(){
    console.log('running on 3002...');
});