Windows.System.winmd

Windows Metadata file Reference

A .winmd file contains Windows Metadata. The format follows the ECMA-335 standard of the Common Language Infrastructure. The file describes the classes and types of a Windows Runtime API. Language projections read the metadata, thus C#, C++, and JavaScript can call the same APIs. Most .winmd files contain metadata only, and the implementation stays in system binaries.

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.