MIME/Format handling should check extensions if an `Accept` header is absent

This would be helpful for developers setting up `.json` routes and wanting to casually test them in local dev.

Locate the MIME/format handling entry point for routes and review how an absent Accept header is currently handled. Verify the behavior for a .json route in local development, with the work complete when the extension is consulted only when no Accept header is present.

Written by the indexing model from the issue text.