According to Microsoft, the ASX metafile extension is used to point to source files. An ASX (Advanced Stream Redirector) file is a metafile that redirects streaming media content away from browsers that are unable to play the content to the Microsoft Windows Media Player. To play an ASX file, Windows Media Player must have a CODEC installed that recognizes the source file’s media type.
The ASX file type is used primarily in conjunction with Microsoft’s Advanced Stream Redirector File, which can pinpoint a Microsoft Advanced Streaming Format, or ASF file, or be included in ASX files.
ASX files store playlists, often used for multimedia presentations or for streaming video files to be played in succession. The ASX files support Real Time Streaming Protocol, or RTSP, as well as Microsoft Media Server, MMS, and HTTP. An ASX file also uses MIME type video/x-ms-asf plug-ins.
Installing a missing or new CODEC is simple. If an ASX file points to a media type the Windows Media Player can’t play, an error message appears telling the user what CODEC to install. These files are usually available for free from Microsoft.