Read multipart/form with AngularJS + SparkJava

Question

I have a client in AngularJS 1.5 and server side using sparkjava 2.5.
I have a post method to send a form that has a json and an attached file.
When I try to read it in the server side, I just can't. I tried everything I searched on the internet and could not figure out a way to read it. Here what happens:

On the client side I tried this approach:

this.createNew = function(file) {
 var file = $scope.newDurability.file
 var fd = new FormData();
 fd.append('file', file);
 fd.append('data', angular.toJson($scope.newDurability));
 $http.post(postURL, fd, {
 headers: {
  'Content-Type': undefined
  },
 transformRequest: angular.identity,
 params: {
  fd
 }
});

And in the sparkjava, I try to read the multipart/form-data by using the code they provide in their webpage:

post("/durability", "multipart/form-data", (req, res) -> {
req.raw().setAttribute("org.eclipse.jetty.multipartConfig",
                        new MultipartConfigElement("/tmp"));
                Collection<Part> parts = req.raw().getParts();
...

But it happens to the parts being always empty.

Also, when I read the body of the request (String body = req.body();), I get this:

------WebKitFormBoundaryrMxfw31B1kgNNhNz
Content-Disposition: form-data; name="file"; filename="Toggl_time_entries_2016-09-01_to_2016-09-30.pdf"
Content-Type: application/pdf
...file binary data...

------WebKitFormBoundaryhHoLHo9P2qmbSNYK
Content-Disposition: form-data; name="data"

{"equipment":"gloves","material":[{"text":"latex"}],"days":1}
------WebKitFormBoundaryhHoLHo9P2qmbSNYK--

When doing the following, everything is either empty or null:

Set<String> attributes = req.attributes();
QueryParamsMap queryMap = req.queryMap();
Set<String> queryParams = req.queryParams();
Map<String, String> params = req.params();

And the same happens when I try to get data by passing a key (it doesn't matter what key I try) to those functions. For example: String params2 = req.params("data"); String parameter = req.raw().getParameter("data");

So the question is: What am I doing wrong? I guess the client side is right, because the request is being sent apparently correct, so my assumption is that there is some trick with sparkjava to make it work. Or maybe, there is a trick to send the form-data in a way that sparkjava will correctly interpret the form and I will be able to read the parts.