I don't know the file extension. I've been trying to find it.
MP says several things very fast and then says "Error downloading codec" and begins playing the audio.
I meant the file extension on the video file you're playing. If that's what you meant, then open a Windows Explorer, click the "Tools" menu, select "Folder Options". Click the "View" tab, find the box labeled "Hide file extensions for known file types", and un-check it if it's checked. Then click the "Apply" button, and "OK" on out. You should be able to view the file extension then.
In Media Player, try right-clicking the file name in the "Now Playing" section, and see if it gives you the option to view the error details.