web.config

Configuration file (.NET) Reference

A .config file contains configuration data, in most cases XML for the .NET Framework. Application configuration files such as app.config and web.config control settings and runtime behavior. The machine.config file holds the .NET settings for the full system. Other software also uses the extension for plain-text settings. A text editor can show the content.

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.