The m3u file name is usually a Windows Media Player Playlist file. If you make a playlist in WMP that is the nametype of file that it uses for it's playlists. You should be able to drag and drop individual mp3 files to your phone. It might not take the playlist though.
I would try to drag a couple of mp3 files to your phone and see what happens.
Perhaps in the past when you dragged and dropped in bulk there was a m3u file amongst the mp3 files and that is why your phone is rejectiing them.