app.xbf

XAML Binary Format file

An .xbf file contains XAML markup in a compiled binary form, the XAML Binary Format. The build process converts .xaml files into .xbf files. The binary form loads faster, because the XML parse step is not necessary at run time. Windows applications with XAML user interfaces ship .xbf files in their packages. The XAML framework of Windows reads the format directly.

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.