"GetFileIcon()" is a transformation method that returns correct markup (not only url) for the icon.
The merkup should look like this:
<i class="icon-file-default icon-file-EXTENSION" aria-hidden="true"></i> you then have to give this markup styling you want it to have. For example:
background-size: 100% auto;
content: " ";
Let me know if the markup you get is different. It is possible without it being a bug. I'll investigate further if you need. However for the 'i' tags you need to provide your own styling or you can link our bootstrap.css along with bootstrap-additional.css, which is not recommended since these are style-sheets for the administration UI and might break your site.