Sony Ericsson

hi, I just got a Sony Ericsson Z310a from Fido. I like it so far, but I downloaded an MP3 to use as a ringtone, from a ringtone website, and the phone won't let me use it. It will play it, and it sounds great, but I cant select it as a ringtone. When I try, it says "Cannot set this file as a ringtune." Does anyone know why? Like I said the phone will play the file, it's 234 kb, 30 secs long. I don't see why it won't, it's kind of annoying, is there a way it needs to be moved to a different file maybe? If anyone knows, I appreciate hearing about it...