Document Records : Cannot Validate Attachment - File Name

We were trying to add a validation rule for the "attachment file name" on "New Document Record" page while adding a document.
Under Configure Validations, we can add validation rules to check fields such as 'Document Name' and 'Document Type' and display an error or warning validation message. 
However, the Attachment's File Name is always NULL!

I checked for:
Filed Values > documentRecords>attachments>File Name : It shows as Null
Filed Values > documentRecords>attachments>Uploaded File Name : It shows as Null
Filed Values > documentRecords>attachmentPreviews>File Name : It shows as Null
Filed Values > documentRecords>attachmentPreviews>Uploaded File Name : It shows as Null