That is a good theory, but I would think that if it was a gas explosion it would tend to blow up quickly while blowing any loose papers or other lightweight stuff up and out. That is also why procedures exist for embassies to burn classified docs when they are being attacked or evacuated. You have to do it correctly though... I have tried burning docs in my smoker after smoking a brisket, and you would be surprised just how many survive even if the fire is really hot. Some get insulated between others if you stack them.