Adding alt-text to an Image in Google Forms API

Question

I am using the google forms api to create a quiz with images. The creation of the quiz works fine, but, I cannot seem to set the alt text for images.

My batchRequest looks like this:

    "createItem": {
      "item": {
        "questionItem": {
          "image": {
            "altText": "foo",
              "sourceUri": "https://pathtomyimage"
          },
            "question": {
              "choiceQuestion": {
                "options": [{
                  "value": "A"
                }, {
                  "value": "B"
                }, {
                  "value": "C"
                }, {
                  "value": "D"
                }],
                  "type": "RADIO"
              },
              "grading": {
                "correctAnswers": {
                  "answers": [{
                    "value": "B"
                  }]
                },
                "pointValue": 1
              },
              "required": true
            }
        },
          "title": "Question 1"
      },
        "location": {
          "index": 0
        }
    }

The altText foo does not appear in the image. Nor as a caption.

In our apps-script based version we were using the addHelpText option for imageItems. See https://developers.google.com/apps-script/reference/forms/image-item