A card JSON is rendering as text

[sunnylbk]

When trying to send a response as below,

{
    "contentType": "application/vnd.microsoft.card.thumbnail",
    "content": {
        "buttons": [
            {
                "type": "Primary",
                "title": "",
                "image": "",
                "value": "Yes"
            },
            {
                "type": "Secondary",
                "title": "",
                "image": "",
                "value": "No"
            }
        ],
        "images": [],
        "subtitle": "Update?",
        "text": "",
        "title": "Start"
    }
}

, it is being rendered as text, rather than as a card with 2 buttons.
Does this require a change in message format type?

Thanks