There is only one file type that uses the .WM extension:
Windows Media File
Windows Media audio or video file; may contain audio, video, or both types of data; formatted using the Windows Media Player codec.
The WM extension is a simplified file extension that can be either a .WMA (Windows Audio) or .WMV (Windows Video) file. The WMA and WMV files are more specific and are used more commonly than the WM extension.
Updated: August 7, 2016