Fixes exception that was thrown during the QUnit test run due to incomplete fixtures

templates/qunit.html

@@ -66,7 +66,7 @@
       <div id="file-list">
         <div class="preview">
           <span class="handle" style="display: none;">&nbsp;</span>
-          <div class="preview-thumb" style="background-image: url();">
+          <div class="preview-thumb" data-url="" style="background-image: url();">
           </div>
           <input type="hidden" id="id_files-0-id" name="files-0-id">
           <span class="js-hidden delete"><input type="checkbox" id="id_files-0-DELETE" name="files-0-DELETE"><label for="id_files-0-DELETE">Delete</label></span>