Integration for: https://github.com/kartik-v/bootstrap-fileinput

See https://github.com/l0rdn1kk0n/wicket-bootstrap/issues/372
Why `ajaxBehavior` is a member field ? 
It seems it is not used out of the constructor.

And why this behavior has to be added to the FileUploadField at all ?
Usually the application decides whether it wants to submit the form with Ajax or not. 
Please remove it from here.