show user dialog when attempting folder drop (#10209)

* adding dialog for folder drop error

* removing debug trace

* polishing wording of dialog

Co-authored-by: Stephanie Stattel <stephaniestattel@Stephanies-iMac.local>

packages/filebrowser/src/listing.ts

@@ -1115,6 +1115,23 @@ export class DirListing extends Widget {
     if (!files || files.length === 0) {
       return;
     }
+    const length = event.dataTransfer?.items.length;
+    if (!length) {
+      return;
+    }
+    for (let i = 0; i < length; i++) {
+      let entry = event.dataTransfer?.items[i].webkitGetAsEntry();
+      if (entry.isDirectory) {
+        console.log('currently not supporting drag + drop for folders');
+        void showDialog({
+          title: this._trans.__('Error Uploading Folder'),
+          body: this._trans.__(
+            'Drag and Drop is currently not supported for folders'
+          ),
+          buttons: [Dialog.cancelButton({ label: this._trans.__('Close') })]
+        });
+      }
+    }
     event.preventDefault();
     for (let i = 0; i < files.length; i++) {
       void this._model.upload(files[i]);