An M3U file is not an actual media file. It is in reality a playlist format, one of the uses of which is to play an MP3 off the net without the MP3 itself being linked to through the browser for download:
http://en.wikipedia.org/wiki/M3U
: : : : To download the (most likely MP3) file that an M3U points to, open up the M3U file in a text editor such as Windows' Notepad, and copy and paste the address of the media file(s) into your browser or a download manager to download it on your computer.
