net/http: DetectContentType does not recognize json

[henrikbjorn]

DetectContentType does not have a sniffer for application/json and therefor fails to guess it. This means it falls back to text/plain.

[mdempsky]

DetectContentType applies the algorithm from https://mimesniff.spec.whatwg.org/#rules-for-identifying-an-unknown-mime-type, which doesn't contain any rules for recognizing JSON.