It's in the option of P2P Client Your using for instance if Your using Lime or Frosty You would look up on the top rite side of that program & click on the Tools menu & then within the drop menu select options, when the options window opens up You would look on the left side for the entry PLAYER & select it,then a new window will open up & You will need to look for a box that says Use Lime/Frostwire Media Player.
Now thats how I enable My P2P Client to allow a Preview