This is used by Google Lens. It will always return the empty string when
dragging an image onto the Google Lens search field.
(cherry picked from commit 535f03b6cf664071e03b7f3c95116fa89c33122b)
This does not implement any of the IDL methods, but GitHub requires the
interface exists to upload files via an <input type="file"> element.
Their JS handles uploads via this element and via drag-and-drop in one
function, and check if the uploaded file is `instanceof DataTransfer` to
decide how to handle it.
