The 264 file extension identifies video encoded with H.264/MPEG-4 Part 10 or AVC (Advanced Video Coding) video codec. However, 264 is not current used file extension for H.264 video, much more are used file extensions like MKV, FLV, MP4, M4V, BSF etc.
If you transcode 264 file to WMV (Windows Media Video) file format, final video quality can be decreased, because WMV format doesn't allow some features exclusive to H.264 codec.