请问用记事本 打开文件 开头出现 MSCF 的是什么文件？

是后缀名为.cab的文件。

In computing, CAB is the Microsoft Windows native compressed archive format. It supports compression and digital signing, and is used in a variety of Microsoft installation engines: Setup API, Device Installer, AdvPack (for the installation of ActiveX components from Internet Explorer) and Windows Installer.

It was originally called Diamond. Its .CAB file extension and overall file format name comes from word cabinet, internally used by Microsoft tools dealing with CAB files to reference such files.