HTML5 drag'n'drop doesn't work with Images

[Ansku]

DropTargetExtensionConnector blocks the drop if the event.dataTransfer contains a type Files. The purpose of that is to keep the listener from consuming the event if an actual file is dragged onto the drop target, so that the same area can optionally be used as a FileDropTarget too. However, the current check accidentally also blocks Image components, as well as some (but not all) other components that contain an <img> tag, like a Label that has an icon set to the caption (but not a Button with the same icon set).

See also https://support.vaadin.com/browse/VS-5045 (internal access only)