cmexcept.cat

Security catalog Reference

A .cat file is a security catalog on Windows. The file contains a list of cryptographic hashes for a set of files, and a digital signature protects the list. Windows compares the hash of a file with the catalog to make sure that the file is genuine and not changed. Driver packages and system updates use catalogs for their signatures. One signature thus protects many files.

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.