On top of using PreviewConfig, you will also have to add entries into the registry to get it working on Windows 7.

• Open the registry editor (regedit in the start menu search) and navigate to computer\HKEY_CLASSES_ROOT\.FILE_EXT, where FILE_EXT is the extension you want to add, such as .nfo

• Make 2 new string values (REG_SZ) under the file extension's key, Content Type and Perceived Type

• Set the value of Content Type to text/plain and Perceived Type to text.