FileDelete.js: add event.preventDefault() to buttonClicked().

javascript/src/FileDelete.js

@@ -26,6 +26,7 @@ var QfqNS = QfqNS || {};
     };
 
     n.FileDelete.prototype.buttonClicked = function (event) {
+        event.preventDefault();
         var alert = new n.Alert("Do you want to delete the file?", "warning", "okcancel");
         alert.on('alert.ok', function () {
             this.performFileDelete(event);