Nokia6020 cellphone can recieve what kind of file extensions?

My fiance got me this cute Nokia 6020 cellphone for my birthday. (I love the man!!!!) I've been trying to download files onto it (mainly songs). I tried the .wav and the .mp3 extensions. They both got downloaded but they wouldn't run saying that the phone doesn't support them. Do you have any idea what extensions I can use? Thanks a lot!!