Allow tree to specify 'files' as drop type (microsoft#153751)

For microsoft#153404 (comment)

This special type indicates that the tree wants to recieve any type of file that is dropped. This matches how the dom drop apis work

Author: mjbvz

src/vs/workbench/browser/parts/views/treeView.ts

@@ -1499,7 +1499,7 @@ export class CustomTreeViewDragAndDrop implements ITreeDragAndDrop<ITreeItem> {
 
 		const outDataTransfer = new VSDataTransfer();
 		for (const [type, item] of originalDataTransfer.entries()) {
-			if (type === this.treeMimeType || dndController.dropMimeTypes.indexOf(type) >= 0) {
+			if (type === this.treeMimeType || dndController.dropMimeTypes.includes(type) || (item.asFile() && dndController.dropMimeTypes.includes(DataTransfers.FILES.toLowerCase()))) {
 				outDataTransfer.append(type, item);
 				if (type === this.treeMimeType) {
 					try {

src/vscode-dts/vscode.d.ts

@@ -10166,6 +10166,8 @@ declare module 'vscode' {
 		 * This includes drops from within the same tree.
 		 * The mime type of a tree is recommended to be of the format `application/vnd.code.tree.<treeidlowercase>`.
 		 *
+		 * Use the special `files` mime type to support all types of dropped files {@link DataTransferFile files}, regardless of the file's actual mime type.
+		 *
 		 * To learn the mime type of a dragged item:
 		 * 1. Set up your `DragAndDropController`
 		 * 2. Use the Developer: Set Log Level... command to set the level to "Debug"