AngularJS how to send multipart/mixed

I am working on a project where I have to upload a small bit of JSON with a file, working in AngularJS.

I've written code using Danial Farid's angular-file-upload, and it is working, except, it always sends "multipart/form-data, boundary=<whatever>"

However, I MUST use multipart/mixed.

This is my call:

$scope.upload = $upload.upload({
  url: <my url>,
  method: 'POST',
  data: $scope.data,
        file: file,
}).progress(function(evt) {
  console.log('percent: ' + parseInt(100.0 * evt.loaded / evt.total));
}).success(function(data, status, headers, config) {
  // file is uploaded successfully
  console.log(data);
});

Is there a way to modify the headers just before it gets sent?

If not using his angular-file-upload, then by another method, hopefully without having to 'roll my own' function?

Edit 1:

I just cannot understand how it can be so difficult to make this change. Of course you can add

headers: {'Content-Type': 'multipart/mixed'}

But this does absolutely NOTHING because there is no boundary. Why can't there be a way to pull the boundary out? Something like

headers: {'Content-Type': 'multipart/mixed, boundary=%b'}

I need to get this working ASAP.

I couldn't wait any longer. I ended up rolling my own and it does work. Here's the code...hopefully others can benefit.

file_contents is the output from reader.readAsArrayBuffer($scope.files[0]);

You need to build up a blob from the preamble text, the file data and the footer, because otherwise appending binary data to a string will do conversions on the binary file data and it will not work properly.

var epochTicks = 621355968000000000;
var ticksPerMillisecond = 10000;
var yourTicks = epochTicks + ((new Date).getTime() * ticksPerMillisecond);

var boundary='---------------------------'+yourTicks;

var header="--"+boundary+"\r\n";

var footer="\r\n--"+boundary+"--\r\n";

var contenttype="multipart/mixed; boundary="+boundary;

var contents=header+"Content-Disposition: form-data; name=\"json\"\r\n";
contents+="Content-Type: application/json\r\n";
contents+="Content-Length: "+JSON.stringify(data).length+"\r\n\r\n";
contents+=JSON.stringify(data)+"\r\n";

contents+=header+"Content-Disposition: form-data; name=\"image\"; filename=\""+$scope.files[0].name+"\"\r\n";
contents+="Content-Transfer-Encoding: binary\r\n";
contents+="Content-Type: "+$scope.files[0].type+"\r\n";
contents+="Content-Length: "+$scope.files[0].size+"\r\n\r\n";

blob=new Blob([contents,file_contents,footer]);

$http.post(restUrl+"user/avatar/uploadAvatar",blob,{'headers':{'Content-Type':contenttype}}).
success(function (data, status, headers, config) {
  alert("success!");
}).
error(function (data, status, headers, config) {
  alert("failed!");
});