title: Polymer Core-Ajax File upload
body: <p>i want to upload a file from a form with core-ajax.
  Right now it my code looks like this:</p>
  
  <pre><code>&lt;core-ajax 
    auto="false"
    method="POST"
    url="/file-upload" 
    handleAs="json"
    on-core-response="{{handleResponse}}"
    contentType="multipart/form-data; boundary=---------------------------7da24f2e50046"
    params="{{item}}"
    id="ajax"&gt;&lt;/core-ajax&gt;
  
  &lt;form&gt;
   &lt;input type="text" name="Name" id="name"&gt;&lt;/br&gt;
   &lt;label class="formLine"&gt;File&lt;/label&gt;&lt;/br&gt;
   &lt;input type="file" name="File" id="file"&gt;&lt;/br&gt;
   &lt;paper-button raisedbutton="" label="send" role="button" aria-label="disabled" class="formLine" on-tap="{{click}}"&gt;&lt;/paper-button&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>with the following javascript-code:</p>
  
  <pre><code>click: function(){
  var name = this.$.name;
  var File = this.$.file;
  
  this.item = {
    'Name': name.value,
    'File':File.value
    };
  this.$.ajax.go();
  }
  </code></pre>
  
  <p>So when i send the request there is no data to process.
  In the previous version i handled this with a regular form and used multiparty to parse the request.</p>
  
  <p>How should i send and handle the data?</p>