Have you uploaded the contact.php file to the widget? You can find that file in the downloadable zip. You can do two things:
1. Upload contact.php to the widget
2. Manually place contact.php in the assets folder. You can find this folder when you export the web in HTML.
There is no way to get rid of the name and email details and merge the upload to your existing form. You can’t do that with this widget or any other widget. You might have to choose which one to use!