onlyJSON.cs

public static void Configure(HttpConfiguration config) {

    config.Formatters.Remove(config.Formatters.XmlFormatter);
    config.Formatters.Remove(config.Formatters.FormUrlEncodedFormatter);

    // From DefaultContentNegotiator class:
    // If ExcludeMatchOnTypeOnly is true then we don't match on type only which means
    // that we return null if we can't match on anything in the request. This is useful
    // for generating 406 (Not Acceptable) status codes.
    config.Services.Replace(
        typeof(IContentNegotiator),
        new DefaultContentNegotiator(excludeMatchOnTypeOnly: true)
    );
}