title: Send FormData with other field in AngularJS
body: <p>I have a form with two <code>input text</code> and one <code>upload</code>. I have to send it to the server but I have some problem concatenating the file with the text. The server expects this answer:</p>
  
  <pre><code>"title=first_input" "text=second_input" "file=my_file.pdf"
  </code></pre>
  
  <p>This is the <strong>html</strong>:</p>
  
  <pre><code>&lt;input type="text" ng-model="title"&gt;
  &lt;input type="text" ng-model="text"&gt;
  &lt;input type="file" file-model="myFile"/&gt;
  &lt;button ng-click="send()"&gt;
  </code></pre>
  
  <p>This is the <strong>Controller</strong>:</p>
  
  <pre><code>$scope.title = null;
  $scope.text = null;
  
  $scope.send = function(){
    var file = $scope.myFile;
    var uploadUrl = 'my_url';
    blockUI.start();
    Add.uploadFileToUrl(file, $scope.newPost.title, $scope.newPost.text, uploadUrl);
  };
  </code></pre>
  
  <p>This is the <strong>Directive</strong> fileModel:</p>
  
  <pre><code>  return {
  restrict: 'A',
  link: function(scope, element, attrs) {
    var model = $parse(attrs.fileModel);
    var modelSetter = model.assign;
  
    element.bind('change', function(){
      scope.$apply(function(){
        modelSetter(scope, element[0].files[0]);
      });
    });
  }
  };
  </code></pre>
  
  <p>And this is the <strong>Service</strong> which call the server:</p>
  
  <pre><code>  this.uploadFileToUrl = function(file, title, text, uploadUrl){
     var fd = new FormData();
     fd.append('file', file);
     var obj = {
       title: title,
       text: text,
       file: fd
     };
     var newObj = JSON.stringify(obj);
  
       $http.post(uploadUrl, newObj, {
         transformRequest: angular.identity,
         headers: {'Content-Type': 'multipart/form-data'}
       })
    .success(function(){
      blockUI.stop();
    })
    .error(function(error){
      toaster.pop('error', 'Errore', error);
    });
  }
  </code></pre>
  
  <p>If I try to send, I get Error 400, and the response is: <code>Multipart form parse error - Invalid boundary in multipart: None</code>.
  The Payload of Request is: <code>{"title":"sadf","text":"sdfsadf","file":{}}</code></p>