If you want to keep it simple, you'd have to have an explorer within a window. You can use the "Microsoft Web browser" ActiveX (see attached Image). At this moment you'd be showing the files somehow in your powerbuilder application, so use the ole control to do that.

To see what I mean: In the constructor event of the OLE control write:

this.object.Navigate("C:\") // for example

Mark the attribute of the ole control "IsDragTarget", though I'm not 100 % sure if that's even needed. It worked for me.

That will give you something like a file explorer, but you still have to tweak it to show side panels etc. Let me know if you need help with that.

From that moment on, you can just drop files from a windows file explorer onto the ole control and vice versa. 

No coding needed at all!

I realise a few things with respect to my previous answer:

1) No one knows if MS is going to keep releasing the WebBrowser control, or if they change it to something we won't be able to use anymore.

2) You'd still have to code your left panel yourself to browse in a treeview through disks and folders (not so hard and there'll be examples).

3) Why re-invent the wheel? Take a look at this https://www.jam-software.com/developer/

They have an activeX (I guess) for Delphi, so that should work with Powerbuilder too.

Not too sure about the licensing. The price per developer seems very cheap, but I don't know if that means free distribution to your customers.