disk cleanup.lnk

Windows shortcut Reference

A .lnk file is a Windows shortcut. The binary file points to a target, for example a program, a document, or a folder. The shortcut stores the target path, the arguments, the icon, and the start folder. The Windows shell shows a shortcut with a small arrow on the icon. Microsoft documents the format as the Shell Link Binary File Format.

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.