title: How can I known how many seconds can a mp3 file play?
body: <p>I want to download song from server, but I don't want to download the whole song, so I'd like to download the first 90‘s ，if user listen the first 80's, I will request the rest of this song. otherwise the rest of this song is unnecessary to download at all.</p>
  
  <p>The request header has a <code>range</code> parameter,I can pass the min - max byte to decide which piece of song I should download.</p>
  
  <p>So, How can I know how many bytes of a song can play 90's??</p>