title: ng-file-upload breaks on Firefox
body: <p>I'm using ng-file-upload in an AngularJS app, it works fine in Chrome and Safari but it breaks in Firefox, I get an
  <code>Error: Argument 2 of FormData.append does not implement interface Blob.</code> message in the console.</p>
  
  <p>Has anyone else had this problem?</p>
  
  <p>The html snippet that contains the directive:</p>
  
  <pre><code>&lt;label class="article-field-label" for="article_description"&gt;Imagens&lt;/label&gt;  
    &lt;div class="file-upload-container"&gt;
      &lt;a class="btn btn-default cancel-file-upload-button" ng-click="cancelPic(1)"&gt;&lt;span&gt;&amp;times;&lt;/span&gt;&lt;/a&gt;
      &lt;div class="button thumbnail-placeholder" ngf-select ng-file-select="onFileSelect($files)"
                    ng-model="article.filesAttachments.file1" name="file" type="file"
                    ngf-pattern="'image/*'" ngf-accept="'image/*'" ngf-max-size="5MB" ngf-min-height="100"
                    ngf-resize="{width: 100, height: 100}"&gt;
      &lt;img ngf-src="thumbnailImage(1)"&gt;
    &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>and my service that makes the POST request to the server:</p>
  
  <pre><code>sendPayload = function(formData, method, url) {
    var options;
    options = {
      url: url,
      method: method,
      file: formData.filesAttachments,
      headers: { 'Content-Type': 'application/json' },
      fields: {
        article: {
          title: formData.title, 
          price: formData.price,
          user_id: formData.user_id,
          category: formData.category,
          article_state: formData.article_state,
          description: formData.description,
          city: formData.city,
          article_id: formData.article_id
        }
      }
    };
    if(method=='POST'){
      return Upload.upload(options).success(function(data, status){
        if(status === 201) {
          var file = formData.filesAttachments
          angular.forEach(file, function(file){
            file.upload = Upload.upload({
              url: '/articles/' + data.article_id + '/attachments.json',
              file: file,
              headers: { 'Content-Type': 'application/json' },
              method: 'POST'
            })
          })
        }
      }).success(function(data, status){
        latestArticleCreatedID = data.article_id;
      }).error(function(data, status) {
        console.log("Failed" + status);
      });
    }
  }
  </code></pre>