title: ngf-src not working with {{vm.detail.id}}
body: <p>I want to display the image preview or the user's avatar when the page is loaded but the <code>ngf-src</code> is not working as I expected:</p>
  
  <pre><code>&lt;img id="avatar" class="ui centered image" ngf-src="avatar || 'http://localhost:8000/img/user_avatar/{{vm.detail.id}}.jpg'"&gt;
  </code></pre>
  
  <p>somehow I can not get the picture from the server.
  It showed on the HTML like this:</p>
  
  <pre><code>&lt;img src="http://localhost:8000/img/user_avatar/.jpg" id="avatar" class="ui centered image" ngf-src="avatar || 'http://localhost:8000/img/user_avatar/5.jpg'"&gt;
  </code></pre>
  
  <p>Could anyone help please!</p>