check if upload form is filled

Hello
i am using vaadin 6 (with liferay). I have some textfilds and a upload component. So theres no problem to validate the textfields, but how can i check, if a user has selected a File.

I have a Button who is sending the form. So the upload will be send with

upload.submitUpload()

So it would be nice to check the upload component befor submitUpload.

Thanks for your help
Peter