mmsys.cpl

Control Panel item Reference

A .cpl file is a Control Panel item on Windows. The file is a DLL that exports the CPlApplet function. The Control Panel loads the file and shows its icons and dialogs. Examples are the classic mouse, sound, and network dialogs. A double-click on a .cpl file opens the item directly.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.