Both file extensions and query parameters are functionally very similar. What I don’t necessarily like about query params for this, is that their usage implies

Both file extensions and query parameters are functionally very similar. What I don’t necessarily like about query params for this, is that their usage implies filtering. It’s a cultural assumption but it’s there. Another point: you might actually have a literal file at a similar path that you serve, typically when you cache responses (managed cache or straight from your app server). Using file extensions gives you a bit more natural affordances. It’s just overal less clunky.