ie9props.propdesc

Property description schema Reference

A .propdesc file describes properties for the Windows property system. The XML file declares custom file properties: name, type, display details, and search behavior. Windows Search and File Explorer read the schema after registration. Applications install .propdesc files to add their properties to the shell. The format is the property description schema.

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.