There is only one file type that uses the .KCL extension:
NIntendo Collision File
A KCL file is a game file used by various Nintendo DS and Wii games, such as Mario Kart. It contains collision data that controls how characters in a game interact with the environment.
Most gamers will not see KCL files or need to open them unless they are looking to modify gameplay. Modders can open KCL files with MKDS Course Modifier and the KCL editor included with CTools. By editing KCL files, modders can change various interactions, such as how the karts interact with the physical constraints of the course (i.e., walls or track floors) or whether a character can walk through walls.
Updated: November 2, 2017