Nintex Form Validate Attachment field

Hello - 

I used SP 2016 On-Premises, I would like to validate the attachments field if empty on the submit form. I used a validation rule below, but none of them are working. How can I validate if attachments empty on the submit form in Nintex New form? I’m appreciated for your help.

not(Attachments)

not({Self})

Attachments < 1

isNullOrEmpty(Attachments)