See RT-12971 for code example.

Open my demo implementation http://testkirkeportal.brandsoft.dk/testapplet.html

and input this URL:

http://www.tizag.com/htmlT/forms.php

Scroll down to find the <input type="file"> example. This doesn't seem to be supported, nothing happens when clicking the button. I don't see any technical reason, why WebEngine shouldn't be able to open a JFileChooser and let the user select a file to upload to the server.