How to get folder thumbnail with alpha channel

There is a nice feature in windows - thumbnail view. So we can see not just folder icon but also small parts of thumbnails of files in the folder. When I tried to get a thumbnail, I got the following: screenshot http://www.freeimagehosting.net/uploads/38515377d7.jpg

C#, Shell api -> native bitmaps -> wpf bitmap(i think this part i开发者_StackOverflows not important)

It is decided by the thumbnail provider currently registered with the file format on the user's system. You can not predict which thumbnail provider would be installed on the client's computer.

I suggest you check the format of the result thumbnail to see if it has the alpha channel.