Right-click the m3u file and choose Open with...., then choose the Notepad.
When the file has opened in Notepad you should see a webaddress where the real MP3 or WMV is hosted. Copy this web address and paste it into your browser address filed and click "Enter". That should bring you to the real mp3 file.
However, if Notepad can't open the file (out of any reason) then I would try to copy the m3u file and paste it into another folder. Then go to this folder and change the ending ".m3u" to "mp3".
That you can change this it might require to go to the folder, open the Folderr Option via Tools>View and click where it says "hide known file extensions (so that it is not marked).
After you've done this you could try to open the file with the media player to see if it still works. Afterwards you could try to upload it again.
