.STSG File Extension
What is an STSG file?
Game file created by SuperTux, a Super Mario Bros. style game except with the Linux penguin "Tux" instead of Mario; stores a saved game and includes progress in a world as well as coins, lives, and score; saved as one of several "slots" available from the in-game save menu.
STSG files are saved in a plain text format and use the filenaming convention slotX.stsg, where "X" is the slot number. The files are stored within the .supertux/save/ directory.