REST Content-Type: Should it be based on extension or Accept header?

Question

Should the representation(html, xml, json) returned by a RESTful web service be determined by the url or by the Accept HTTP header?

Answer 1

There are problems with using content type... I discussed this on my blog http://shouldersofgiants.co.uk/Blog and finally settled on including the representation in the URI as suggested in RESTful Web Services by Richardson and Ruby