"If an OS had to parse the file header of every file it looked at it would run slower than molasses in January."
Crikey! I'm so glad my A/V suite doesn't scan the entire contents of everything I write to disk looking for known or similar to known byte sequences according to a heuristic detection algorithm, recursing archives as necessary My machine would be waaaay too slow to be usable then.
Oh, wait. It does and it isn't.........