Update Header with application/json (#21047)

web/web.go

@@ -61,8 +61,8 @@ func Handle404(a app.AppIface, w http.ResponseWriter, r *http.Request) {
 	err := model.NewAppError("Handle404", "api.context.404.app_error", nil, "", http.StatusNotFound)
 	ipAddress := utils.GetIPAddress(r, a.Config().ServiceSettings.TrustedProxyIPHeader)
 	mlog.Debug("not found handler triggered", mlog.String("path", r.URL.Path), mlog.Int("code", 404), mlog.String("ip", ipAddress))
-
 	if IsAPICall(a, r) {
+		w.Header().Set("Content-Type", "application/json")
 		w.WriteHeader(err.StatusCode)
 		err.DetailedError = "There doesn't appear to be an api call for the url='" + r.URL.Path + "'.  Typo? are you missing a team_id or user_id as part of the url?"
 		w.Write([]byte(err.ToJSON()))