According to the following KB article this is a feature not a bug.
The claim is that by limiting searches to known file types the searches will be faster.
Faster yes, but also pointless as the files that I am actually searching for are ignored.
This should have been controlled by a simple registry setting with the default being the existing windows 2000 behaviour.
It appears that the drone that designed, coded and tested this has never actually worked outside of a software house.
Big hint: the real world has far more file types than those known by microsoft!