defaultwindows_supplemental.xml

XML document Reference

An .xml file contains structured data in the Extensible Markup Language. The format stores elements and attributes as plain text, and both software and persons can read it. XML is a standard of the W3C. Many configuration and document formats build on XML. A schema can define and validate the permitted structure.

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.