Form Actions > Validate > Uploded File Missing

Form Actions > Validate > Uploded File Missing

Hello,
Since I cannot set hidden filelds as Mandatory, the only option I have is to "Notify on Validation".
How can i write this on the Workflow?

I need to write that:

if field1 is "Yes"
then
field2 must contain an uploaded file

how do i write this?

Thanks,
Tor Vergata University