Cache REST

Perhaps you are referring to the ContentType response header which is used by many applications to know what format the response is in.  In one of my RESTful services that returns a JSON response I use the following code in my REST handler.

    Set %response.ContentType="application/json"

You can find a list of all of the various MIME types that could be used as the %response.ContentType

https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Complete_list_of_MIME_types