If you are a user of Microsoft SharePoint Technologies, you may notice that Microsoft only supplies icons for Microsoft or generic products in their Document Libraries. For instance, PDF documents show up with the default windows icon and the extension is hidden. So any user hasn't a clue what type of file it is until she clicks on it. I found this site to be informative and accurate to add the PDF icon into a location on your server hosting SharePoint and where to edit an XML file to add the file to SharePoint. This fix will work for any document extension you wish to add an icon to SharePoint. Once the changes are added you will need to restart your IIS services or reboot the server.
The site also includes an icon and batch file to automate the process, but use at your own risk.