Use file.url when file.thumbnailURL not available

If the server does not support imageMagik, there will be no thumbnailUrl.
A workaround is to use file.url.

```html
{% if (file.thumbnailUrl) { %}
      <img src="{%=file.thumbnailUrl%}"></a>
{% } else { %}
      <img src="{%=file.url%}" class='img-responsive'>
{% } %}
```