Instead of adding another conditional (nesting) to ignore thumbs.db, we should just add that check to the already existing checks against sprites.png and sprites_ie6.gif.  so the code should be:

if (file.Name.ToLowerInvariant() == "sprites.png".ToLowerInvariant() 
   || file.Name.ToLowerInvariant() == "sprites_ie6.gif".ToLowerInvariant()
   || file.Name.ToLowerInvariant() != "thumbs.db".ToLowerInvariant())
   continue;