[michaelmior]

> on other platforms the convention is to look at the file contents

MacOS (that is, Finder) also looks at the extension. That has also been the case with any file manager I've used on Linux distros that I can recall.

[jdiff]

You might be surprised. Rename your Photo.JPG as Photo.PNG and you'll still get a perfectly fine thumbnail. The extension is a hint, but it isn't definitive, especially when you start downloading from the web.