When dragging an object from a webpage that is set up as a draggable object, then Qt should see the extra mime types if any are provided.

For example, if you drag the image in the box from https://www.w3schools.com/html/html5_draganddrop.asp and drop it into the dropsite example (in qtbase/examples/widgets/draganddrop) then it should be indicating one of the mime types available as being an image. But it only lists text, html and uri-list. Comparing to dragging from Chrome into the dropsite example the image is available as data.

There are other cases where you can drag something providing JSON data too, but again Qt only gets a QDrag that provides text, html, and uri-list. The custom data should also be available.