Problem of www

Hi guys ,I've progamming a litle game using unity3D, in that game,we need download some image or text from internet, but is not such goodidea if we download those asstes every time,so I work on some tools call Download Manager, function was when I download somthing I will create a local file, when next time I need to Download same Url , i'll using www to load local file ,long story short，when I try to load local file form android system by using class www , every time I got a error like this:


here is my code:


THX for your watching and hope you can help me !

 

You can use this function:
WWW.LoadFromCacheOrDownload
http://docs.unity3d.com/ScriptReference/WWW.LoadFromCacheOrDownload.html

 

it works thank u