The FileUploadField will close any input streams you have opened in its
FileUpload by default. If you wish to manage the stream yourself (e.g.
you want to use it in another thread) then you can override this method
to prevent this behaviour.
Clean up at the end of the request. This means closing all inputstreams
which might have been opened from the fileUpload.
See Also:org.apache.wicket.Component.onDetach