Click on ‘Add to shape area’. After that click on Expand.
Fill it with Gradient and set Stroke on ’11 pt’.
Copy the object and paste it. Make something like in the picture with the Pen Tool on the pasted object
Select both objects and click on Divide. Select the bottom part with the Direct Selection Tool and delete it.
Go to Object > Path > Offset Path. Delete the outside part.
Place it on the original object and give it a black/white gradient. Set it on Screen.
When you are done, you should have a Folder Icon that looks similar to this.