we are glad to know QB support json format from v10
we try the json format result, but here catch a different behavior with XML format
with default header, we will receive the 'Content-Type' is 'application/xml' or 'text/plain'
But when we try the json header, just receive 'application/json'

So the problem is access some 'text/plain' like 'rest/version' receive type also is 'application/json'