Pass in readStream to a file upload request in client-side Javascript

I am trying to fetch an image and read it as a stream. Once I have the stream, I want to use this stream to upload the image to a server.

fetch('http://localhost:8080/dist/boo.jpeg')
          .then(response => response.body)
          .then( readStream => {
            fetch('http://localhost:8080/upload', {method: 'post', body: readStream});  
          });

or

fetch('http://localhost:8080/dist/boo.jpeg')
          .then(response => response.body)
          .then( readStream => {
            var xhr = new XMLHttpRequest();
            xhr.open('POST', '/upload', true);
            xhr.overrideMimeType('multipart/form-data');
            xhr.send(readStream);
          });

But the request payload that's sent is a string - [object ReadableStream]