Possible to restrict deleting attachment in Document Records?

Summary:

Desired business process:

• HR creates placeholder offer via Create Job Offer
• HR extends and accepts placeholder offer on behalf of internal candidate
• Placeholder offer is uploaded to Document Records
• HR navigates to Document Records to upload and attach the real internal offer letter
• HR is unable to delete any attachments within the Document Record submission (i.e. historical attachments will remain)

We configured Restrict Delete, but this only restricts the user from deleting the record (removes Delete button). The user can still delete an attachment and save the record. 

We configured Restrict Update, but this removes the Edit button so the user is unable to edit the record to add a new attachment.