[Plugin: WordPress Form Manager] How to make upload file element required?

I have a form where a file to upload must have been chosen before the form is submitted.

There is no ‘required’ check-box on the file element, so is this possible?

I tried setting a rule so the rest of the form was hidden until the file had been chosen, but this doesn’t work – the hidden elements never appear.

https://www.ads-software.com/extend/plugins/wordpress-form-manager/