I've received a couple of requests lately about how to implement drag-and-drop multiple file uploads in Internet Explorer from within managed code. Fortunately for me, someone else has already taken the liberty of writing an article on the subject at large: Host Secure, Lightweight Client-Side Controls in Microsoft Internet Explorer. Just a couple of tweaks and it'll be accepting files via drag and drop too.
In MultiUploadCtrl.cs, we need to enable drag and drop processing in the user control. Setting AllowDrop=True handles that in one swoop, now all that's left is the plumbing to handle the dropped files. For the events, DragEnter and DragDrop, the following should work well: