There is no effective way to track attachment opens. Even Microsoft's own interface, documentation, & marketing is misleading and troubling.

There ought to be a way to track attachment opens in which, when one is marked as follow, it is uploaded to the sender's OneDrive and a link automatically added to the body of the email. That way "Attachment Views" will again have value in user Activity in Dynamics 365 Sales. Currently, it's orphaned and causes endless headaches to explain this, further causing frustration with the platform and friction between partners and Microsoft's customers.