How to use metadata to view Uploaded File by the multer-gridfs-storage and Node.js

Question

I have uploaded a PDF file and its metadata UserId with multer-gridfs-storage and Node.js, and I can query files out by const file = gfs.find({ metadata: { UserID: req.params.UserId } }), console.log (files) shows the files.

But I got problem to display the file with

const readstream = gfs
    .openDownloadStreamByName(req.params.filename)
    .pipe(res); ......

The partial code is as

const storage = new GridFsStorage({
    // url: (mongoURI, { useUnifiedTopology: true}),

    url: mongoURI,
    options: { useNewUrlParser: true, useUnifiedTopology: true },
    file: (req, file) => {
        return new Promise((resolve, reject) => {
            crypto.randomBytes(16, (err, buf) => {
                if (err) {
                    return reject(err);
                }

                const filename = buf.toString("hex") + path.extname(file.originalname);

                const fileInfo = {
                    filename: filename,
                    bucketName: "uploads",
                    metadata: req.body,
                };

                resolve(fileInfo);
            });
        });
    },
});

const upload = multer({ storage });

//-------------------------------------------Query files------

router.get("/pdf/:UserId", (req, res) => {
    const file = gfs
        .find({ metadata: { UserID: req.params.UserId } })
        .toArray((err, files) => {
            // Check if file
            if (!files || files.legnth === 0) {
                return res.status(404).json({
                    err: "No file exists",
                });
            }
            console.log({ files });   // Yes, work OK. how to extract files to get the filename ?

            const readstream = gfs
                .openDownloadStreamByName(req.params.filename)  // code for other query
                .pipe(res);
        });
});

How to get filename from files { }object to replace req.params.filename?