Fix FileInput widget styling

http://getbootstrap.com/css/#forms-example

Bootstrap 3 does not suggest using .form-control on file selection widgets. Doing so messes up the look of the widget.

Example.

![input-with-form-control](https://f.cloud.github.com/assets/46186/1086261/aea8c406-15f7-11e3-8056-b9ea29fea7f3.png)