Read the body of Node.js response from client

I'm sure this has a simple answer, but for the life of me I can't figure out how to do it.

I have the following express endpoint for uploading to Google Cloud storage. It works great and the response from the google api gives me a unique file name that I want to pass back to my front end:

app.post('/upload', (req, res) => {
  var form = new formidable.IncomingForm(),
  files = [],
  fields = [];

  form
    .on('field', function(field, value) {
      fields.push([field, value]);
    })
    .on('file', function(field, file) {
      files.push([field, file]);
    })
    .on('end', function() {
      console.log('-> upload done');
    });
  form.parse(req, function(err, fields, files){
    var filePath = files.file.path;
    bucket.upload(filePath, function(err, file, apiResponse){
      if (!err){
        res.writeHead(200, {'content-type': 'text/plain'});
        res.end("Unique File Name:" + file.name);
      }else{
        res.writeHead(500);
        res.end();
      }
    });
  });

 return;
});

I reach this endpoint by calling a short function which passes the file to it:

function upload(file) {
  var data = new FormData();
  data.append('file', file);
  return fetch(`upload`,{
    method: 'POST',
    body: data
  });
}

const Client = { upload };
export default Client;

This function is called from my front end like this:

Client.upload(this.file).then((data) => {
  console.log(data);
});

This final console.log(data) logs the response to the console. However, I don't see anywhere the response that I wrote in ("Unique File Name:" + file.name)

Does anyone have any suggestions for how I can retrieve this info from the response body on the client side?

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