How to Automatically set the featured image. for my file uploads

Hi, I have been using wordpress 3.4.1 I am making my site as notes sharing, where users can uploads there pdf , doc files, and videos. The only problem I am facing is when user is uploading any file he need to set the featured image as a thumbs image. i just want that when any user uploads any specific file like pdf or doc the custom icon will automatically set as a featured image for that particular uploads. (For Example for pdf upload a pdf icon and for doc upload a doc icon.)
Is there any plugin is available for making uploads type icons. Or suggest me other way to do this Please help me for this……..