There is a step up from diodes called [inspecting] data guards and an adjacent technology called content disarm and reconstruct (CDR) that doesn't rely on signatures or heuristics - it just assumes every document is malicious.

Combining these 3 technologies with certain policies, e.g. 2 man rule, the hw/sw itself developed on airgap you can make it practically impossible to attack, even for nation state adversaries.

Edit to point out that these all work in 2-way configurations as well.