arcgis javascript api 4 - Displaying media (JPG) in Popup for Feature Layer?

var featureLayer = new FeatureLayer({
      url: "https://services.arcgis.com/8MqUfIH3ilrffIf0/arcgis/rest/services/aneityum_SPEED_example_30May2017_geo/FeatureServer",
      outFields: ["*"],
      popupTemplate: {
        title: "{ipa}",
        content: "
Definition: {gloss}
" + "
Definition provided by: {authority}
",
        type: "media",
        mediaInfos: [{

          title: "test",
          //caption: "DEF: {gloss}\n Definition provided by: {authority}",
          type: "image",
          value: {
            sourceUrl: "{image_link}"
            //linkUrl: "{image_link}"
          }
        }]
      }
    });

I'm trying to put images into my popup (using ArcGIS Javascript API), the links to which are included within the GeoJSON file in the map server. However, the media feature does not seem to be working––the "test" title is not even showing up. Any ideas as to why?