What is a TNL file? A TNL file is a thumbnail image created by Super Mario Maker, a game for the Wii U. It is used to show a thumbnail of an associated course in-game. Each course's TNL files are stored in that course's directory, alongside the course's .CDT and .BWV files.

More Information Super Mario Maker is an official Super Mario level creator available for the Wii U. It allows players to create, play, and share Super Mario levels (commonly referred to as courses) with others. Super Mario Maker saves each of a player's courses in its own specific directory, on their Wii U's hard drive. Most players will never encounter these directories, but modders and other Super Mario Maker enthusiasts can use various utilities to extract and view their Super Mario Maker save data. When they do, they'll see each course saved in its own directory (named course### ), which includes the following files: A CDT file that contains most of the course's data A BWV file that contains the course's music Two TNL files that contain course thumbnails Each TNL file is a .JPG file that includes a proprietary 8-byte file header. If you remove this header, you can view the thumbnail the TNL file contains as you would any JPG file.