mmcss.sys

Windows system file Reference

A .sys file is a Windows system file, in most cases a kernel-mode driver. The file has the Portable Executable format. The Windows kernel loads the driver to operate hardware and system services. Modern Windows versions accept only drivers with a valid digital signature. Examples are storage, network, and file system drivers.

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.