I dont know my German, and google-translate did an awful job BUT...

You seem to be after a thumbnail-viewer for windows 7

Im using SageTools 2.0.0.12 with my Windows Professional 64bit, also using Explorer 64bit version.

It gives me all formats I can think of as thumbnails. All TGA's and 'most' DDS's.
(By 'most' DDS's I mean I have ONE self-created 16bit float DDS that wont preview...)

Unfortunately I cant remember where I downloaded it from,
but I THINK it was SourceForge...

Hope this is of some help...