WindowsUpdate.admx

Group Policy administrative template Reference

An .admx file is a language-neutral administrative template for Group Policy. The XML file defines policy settings, their registry locations, and their possible values. The Group Policy editor reads .admx files and builds its settings tree. The display strings come from companion .adml files, one for each language. Administrators can put .admx files into a central store of the domain.

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.