$upload.upload({
  url: "/upload",
  method: 'POST',
  data: data,
  file: file,
  formDataAppender: function(formData, key, value){
    // key => 'stuff'
    // value => { 'foo': 'bar' }
    for (k in value) {
      var dataKey = key + "[" + k + "]"; // => "stuff[foo]"
      formData.append(dataKey, value[k]); // => sets "stuff[foo]=bar" - the correct way to send the form data
    }
  }
})