Trying to send a zip from the backend to the frontend

At one point in my ember.js webapp the user is given the option to download a zip file. On clicking the button, an action is triggered that sends a request to the backend server, which generates the zip and returns it. Ideally the zip should then be downloaded automatically.

In my backend endpoint I return with

return Response
        .ok(FileUtils.readFileToByteArray(new File(tmpZipFilename))) // tmpZipFilename is a String
        .type("application/zip")
        .header("Content-Disposition", "attachment; filename=\"" + finalZipFilename + "\"")
        .build();

In the frontend I have (adapted from here)

submit() {
  var formData = new FormData(this);
  let token = this.get('session.data.authenticated.token');
  jquery.ajax({
    url: `myUrl`,
    data: formData,
    processData: false,
    contentType: false,
    beforeSend: function(xhr) {xhr.setRequestHeader('Authorization', `Bearer ${token}`)},
    type: 'POST',
    success: function(data) {
      var blob = new Blob([data], {type: 'application/zip'});
      let a = document.createElement("a");
      a.style = "display: none";
      document.body.appendChild(a);
      let url = window.URL.createObjectURL(blob);
      a.href = url;
      a.download = 'myFile.zip';
      a.click();
      window.URL.revokeObjectURL(url);
    },
    failure: function() {
      // other stuff
    }
  })
}

The response headers are the following:

HTTP/1.1 200 OK
X-Powered-By: Undertow/1
Cache-Control: no-store
Date: Tue, 19 Feb 2019 16:34:35 GMT
Server: WildFly/10
Content-Type: application/zip
Content-Disposition: attachment; filename="filename.zip"
Connection: close
Transfer-Encoding: chunked

I have confirmed that tmpZipFilename in the backend section does correctly point to a proper zip file. And when the user clicks the download button a file called myFile.zip is indeed downloaded. However, the file downloaded is not unzippable and is a different size than the correct file pointed to by tmpZipFilename. What am I doing wrong?