If you cannot see the m3u extention, do this:
To show filetypes, Ctrl+Click or right click an m3u file and choose Get Info. There is an option to hide or show the extension.
If you can see it, right click or ctrl+click the file and choose Rename, then erase the m3u and write in .mp3, accept the warning to change it to mp3.
Keep my original advice in mind however, re: the files being playlists and not actually containing audio, if they refuse to play after doing this.