I agree that I've simplified things a bit, 'file' is rather the user facing tool that does the same kind of inspection, and of course the X bit is a prerequisit

I agree that I've simplified things a bit, 'file' is rather the user facing tool that does the same kind of inspection, and of course the X bit is a prerequisite. I know this but I didn't want to write a whole elaboration on my phone :) But my main point was that extensions aren't really a thing used at OS level to determine file type like on Windows, like the OP said.