PreviewConfig is a very useful utility written by Microsoft MVP Ramesh Srinivasan that allows users to easily register file types with the Windows Explorer file preview pane.
Start PreviewConfig and you can view all of the currently registered preview filters on your system for the different file types. Adding a new file type to register a preview for is as easy as just typing in the extension and then selecting the type of viewer.
PreviewConfig works great for text based files that windows does not recgonize by default. For example, .NFO or programming language files such as .vb or .cs.