Pure jsinterop example showing how to drag and drop file and upload it - see https://colinalworth.com/dnd-j2cl/ for running demo (minus actual upload)

ElementalFileDndUploadExample.java

import com.google.gwt.core.client.EntryPoint;
import elemental2.dom.DataTransfer;
import elemental2.dom.DragEvent;
import elemental2.dom.Element;
import elemental2.dom.File;
import elemental2.dom.FileList;
import elemental2.dom.FormData;
import elemental2.dom.HTMLDivElement;
import elemental2.dom.XMLHttpRequest;

import static elemental2.dom.DomGlobal.document;
import static elemental2.dom.DomGlobal.window;

/**
 * https://developer.mozilla.org/en-US/docs/Web/API/HTML_Drag_and_Drop_API/File_drag_and_drop
 * 
 * https://github.com/google/elemental2
 * 
 * <inherits name="elemental2.core.Core"/>
 * <inherits name="elemental2.dom.Dom"/>
 */
public class FlowChartEntryPoint implements EntryPoint {
    @Override
    public void onModuleLoad() {
        Element divElement = document.createElement("div");
        divElement.setAttribute("style", "border: 1px dashed red; width: 300px; height: 300px;");
        divElement.addEventListener("dragover", evt -> {
            window.console.log("dragOver");
            evt.stopPropagation();
            evt.preventDefault();

            DragEvent dragEvent = (DragEvent) evt;
            DataTransfer dataTransfer = dragEvent.dataTransfer;
            dataTransfer.dropEffect = "copy";
        });

        divElement.addEventListener("drop", evt -> {
            window.console.log("drop");
            evt.stopPropagation();
            evt.preventDefault();

            DragEvent dragEvent = (DragEvent) evt;
            DataTransfer dataTransfer = dragEvent.dataTransfer;
            FileList files = dataTransfer.files;
            for (int i=0; i < files.length; i++) {
                File file = files.item(i);
                window.console.log("file: " + file.name);

                upload(file);
            }
        });

        divElement.addEventListener("dragEnd", evt -> window.console.log("dragEnd"));

        HTMLDivElement wrap = (HTMLDivElement) document.createElement("div");
        wrap.style.setProperty("border", "1px solid purple");

        wrap.appendChild(divElement);

        document.body.appendChild(wrap);
    }

    protected void upload(File file) {
        FormData formData = new FormData();
        formData.append("file", file);

        XMLHttpRequest xmlHttpRequest = new XMLHttpRequest();
        xmlHttpRequest.open("POST", "/");
        xmlHttpRequest.send(formData);
    }
}