msdvbnp.ax

DirectShow filter Reference

An .ax file is a DirectShow filter on Windows. The file is a DLL in the PE format with COM interfaces. Media applications connect such filters into a chain that decodes or processes audio and video. Windows ships filters for common media formats as .ax files. The regsvr32 tool registers a filter with the system.

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.