quite black thumbnails on multi layer XCF files with transparency in the layers

What steps will reproduce the problem?
1. create a GIMP xcf file with multiple layers
2. add alpha channel to most of the layers or tune layer overall opacity to a 
medium value
3. use the rubber tool to add more alpha effects
4. see whats the thumbnail in explorer thumbnail view

What is the expected output? What do you see instead?
the thumnail should roughly look like the image in the file, the thumbnail does 
look mostly black and thus is neither amazing nor very useable.

What version of the product are you using? On what operating system?
2.0.0.14 - OS WnVista HP 32 bits

Please provide any additional information below.
GIMP v2.6

Original issue reported on code.google.com by Alexande...@gmx.de on 21 Feb 2012 at 10:28

TomTGS / sagethumbs

quite black thumbnails on multi layer XCF files with transparency in the layers #18