The Secret Files (TSF) is an add-on for Jazz Jackrabbit 2 which was released in 1998. It features 3 new tilesets, 9 new levels, a new character named Lori Jackrabbit, support for much larger tilesets (4096 tiles instead of 1024), two new enemies, and a new version of the engine: 1.24. 1.23 levels can be loaded by The Secret Files, but 1.23 cannot run TSF levels. People using the 1.24 engine also cannot join 1.23 games (and vice-versa) unless the game is tricked into thinking it runs another version of the engine by editing the version number, usually with a HEX editor.
The Secret Files featured one new playable character, Lori Jackrabbit. Lori is the sister of Jazz and Spaz. There has been some confusion about her status in relation to the Jackrabbit family. Originally she was intended to be Spaz’s girlfriend, but that was changed by accident.
For more information, see the Lori Jackrabbit article.
It is hard to find The Secret Files in stores at the moment. It is currently only occasionally sold in Germany, Poland, and the Netherlands. Because of the availability of the game in the Netherlands and Poland, there are a lot of users from these nations on TSF, which has led to Polish and Dutch being languages that are spoken a lot on TSF servers, while the primary language for servers from the original game is usually English.
Eat your lima beans, Johnny.