update.mum

Microsoft Update Manifest

A .mum file is a Microsoft Update Manifest. The file contains XML metadata for a servicing package of Windows Update. The component servicing system reads the file when it installs or removes an update. The metadata names the package, its version, and the components in it. These files stay in the servicing store of Windows.

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.