angularjs ng-flow message not show up

Question

I am currently working on the ng-flow plugin, the code below is copy from quick setup for ng-flow (website reference: https://github.com/flowjs/ng-flow)

<div flow-init="{target: '/upload'}"
     flow-files-submitted="$flow.upload()"
     flow-file-success="$file.msg = $message">

  <input type="file" flow-btn/>
  Input OR Other element as upload button
  <span class="btn" flow-btn>Upload File</span>

  <table>
    <tr ng-repeat="file in $flow.files">
        <td>{{$index+1}}</td>
        <td>{{file.name}}</td>
        <td>{{file.msg}}</td>
    </tr>
  </table>
</div>

The $index+1 and file.name is showing up, however the file.msg does not show anything. Can anyone explain this problem to me?