> How can a shell do anything serious with asserted file types that can be missing, wrong, malformed, unknown or mendacious? It's hard to go beyond offering a best-effort "open with the associated program" application as an alternative to vi, cat etc.

How would that be any worse than the status quo, though?

Programs already output, depending on their invocation and the utility at hand, binary data in arbitrary formats, shell escape sequences, text, ASCII art, etc.

The only issue I can think of is for portability of the scripts, if a program gets updated to support a new type as input or output, and that causes issues compared to the testing environment. But that's arguably already an issue too, and it would be rather simple to annotate expected types.