Why aren't REST (JSON) responses using Content-Type: application/json?

Question

Working with the REST API and I noticed that the content-types are not being set correctly.

As an example:
ESRI_Census_USA Sample Service: 
http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Demographics/ESRI_Census_USA/MapServer?f=json

returns: 
Content-Type:text/plain;

Is there configuration or URL parameter to fix this?

MuratAkdogan · Answer

we also have the same doubt. please add wfs:outputFormat=application/json?

LeonAus · Answer

False alarm. The rest directory automatically uses f=pjson as the format which returns text/plain. Changed to f=json returns the application/json needed.

LeonAus · Answer

Marc,

My ArcGIS server install is 10.6.1 and this change doesn't seem to be working. By default I still get text/plain when requesting a json response. I tried to add "defaultJsonResponseContentType": "application/json" into my system properties as a long shot but that didn't make a difference.

Is there anything else that is needed to get the response in application/json or is this a bug and I should log it with my local distributor?