What more would you like to know?
As a folder (e.g. RealFile, DVDISOFile etc.) is also a DLNAResource, it has the possibility to override getThumbnailInputStream(). Then its up to the renderer to show it (or not..). I guess sony decided to always show the same icon for folders in order to see at one glance what is a file and what is a folder!?
I haven't got any experience with other renderers, but assume some will show the thumbnail for a folder if it has been set (correct me if I'm wrong