@anderj39 .....I'm not sure if File Upload have the option of Minimum Attachments in Validation or not?
In on-prem attachment control do have the option to provide minimum attachments and maybe using run time formula we can set the value to be either 0 or 1 based on AttachmentDD value.
But if that's not the option then instead of using your formula try to use If statement in validation rule of attachment control
If(equals(AttachmentsDD, "Yes"), If(isNullOrEmpty(ItemProperty:Attachments), "Please attach files", false), false)
I dont think ItemProperty:Attachments will work on a new form as the item would not have been created so therefore this would always come up as zero or null.
Was this ever solved, I have the same issue. Need to conditionally require attachments.