I wonder if it's really safer. You'd probably find that you'd have different magic databases and a carefully constructed (malicious) document might give differ

I wonder if it's really safer. You'd probably find that you'd have different magic databases and a carefully constructed (malicious) document might give different answers. Imagine that your email client decided a document was a PDF, but your file manager thought it was a loadable extension. You're trading one class of vulnerability for another. I'd also argue that the endurance of the file extension is a hint that people may actually like them. Extensions were invented as an affordance to the computer (which it no longer needs) but the information they convey is useful to humans as well. It may be a quirky old convention, but it provides a universally-understood language for describing a file type. Anyway, I think the overall discussion on this submission is a good example of why 3-character extensions survive: some people here argue for longer extensions, others say we should have no extensions, and the silent majority is happy to keep splitting the difference.