Problem while sending file between 2 servers – Javascript

I want to send a file from a server to an other.
The problem is that my second server (who receive) don’t read correctly my file.
Normally I send my file to an API but I create a little server to make my own tests.

My server (sender):

First I got my file with multer().single('file')
(My file only contain "Hello")

const formData = new FormData();
    const file = new File(req.file.buffer, req.file.originalname, { type: req.file.mimetype })
    formData.append("file", file);

    let firstFile = null;
    for (const [key, value] of formData.entries()) {
        if (value instanceof File) {
            firstFile = value;
            break;
        }
    }

    const uploadres = await fetch(url, {
        method: "POST",
        headers: {
            "Authorization": "Bearer " + token,
            "Content-type": "multipart/form-data"
        },
        body: firstFile
    })

Then I read the file like this :

var body;

    body = '';
    req.on('data', function (chunk) {
        body += chunk;
    });

    req.on('end', function () {
        console.log(req.method + ' ' + req.url + ' HTTP/' + req.httpVersion);

        for (prop in req.headers) {
            console.log(toTitleCase(prop) + ': ' + req.headers[prop]);
        }

        if (body.length > 0) {
            if (req.headers['content-type'] === 'application/x-www-form-urlencoded') {
                body = decodeURIComponent(body)
            }
            console.log('n' + body);
        }
        console.log('');

        res.writeHead(200);
        res.end();
    });

my server print: "72101108108111"

I know that something was wrong in my code because the API receive my file but don’t doesn’t perform normally.

I think my problem become from encoding but I don’t know how to fix that.

I have try a lot of things, like change my file with another etc. but nothing works.