Windows Media Player 11 SDK Metafile Extension Guidelines
A file name extension provides an independent software vendor (ISV) with information about the rendering requirements of an application that uses the extension, and enables content authors to target general types of players.
Windows Media metafile extensions are used to identify the format of the Windows Media files that a metafile references. Windows Media metafiles with .wax, .wvx, or .asx extensions reference files with .wma, .wmv, and .asf extensions, respectively. All metafiles, regardless of the file name extension used, have the ASX element tag at the beginning of the file with the version attribute specified.
The following table shows the media file types referenced by each type of metafile file name extension. The columns list media file extensions, the rows list metafile extensions. An X in a column indicates a media file type that can be referenced by a particular metafile file name extension.