GWT Elemento file drop onto a browser div and then upload the file example.

ElementoFileDndUploadExample.java

package dnd;

import static elemental2.dom.DomGlobal.document;
import static org.jboss.gwt.elemento.core.EventType.dragend;
import static org.jboss.gwt.elemento.core.EventType.dragover;
import static org.jboss.gwt.elemento.core.EventType.drop;

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.core.client.GWT;
import elemental2.dom.DataTransfer;
import elemental2.dom.File;
import elemental2.dom.FileList;
import elemental2.dom.FormData;
import elemental2.dom.XMLHttpRequest;
import org.jboss.gwt.elemento.core.Elements;

/**
 * https://developer.mozilla.org/en-US/docs/Web/API/HTML_Drag_and_Drop_API/File_drag_and_drop
 *
 * https://github.com/hal/elemento
 *
 * <inherits name="org.jboss.gwt.elemento.Core"/>
 */
public class ElementoFileDndUploadExample implements EntryPoint {

    @Override public void onModuleLoad() {
        document.body.appendChild(Elements.div()
                .style("border: 3px dashed grey; width: 300px; height: 300px;")
                .textContent("Drop files here!")
                .on(dragover, evt -> {
                    GWT.log("dragOver");
                    evt.stopPropagation();
                    evt.preventDefault();

                    DataTransfer dataTransfer = evt.dataTransfer;
                    dataTransfer.dropEffect = "copy";
                })
                .on(drop, evt -> {
                    GWT.log("drop");
                    evt.stopPropagation();
                    evt.preventDefault();

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

                        FormData formData = new FormData();
                        formData.append("file", file);

                        XMLHttpRequest xmlHttpRequest = new XMLHttpRequest();
                        xmlHttpRequest.open("POST", "/");
                        xmlHttpRequest.send(formData);
                    }
                })
                .on(dragend, evt -> GWT.log("dragEnd"))
                .asElement());
    }
}