All those special syntaxes only trigger if args.length >= 2,
so first we can nest them in a new scope, which also makes
it visually more distinct.

Additionally, the third and second cases were actually inverted:
if the third case was to pass, it means the second case couldn't
have passed (because they check present/absence of an extension,
respectively), so we can swap them and use an else-if.

The result is arguably much more readable, and highlight the fact
that if the first case (stdin) is used, the second case will.