By default when you deploy a Silverlight application to an IIS 6.0 server and try to load the page, you will get a JavaScript error:

Error: Unhandled Error in Silverlight Application
Code: 2104   
Category: InitializeError      
Message: Could not download the Silverlight application. Check web server settings

To fix this:

  1. Open IIS 6.0 Manager
  2. Right click the web site that will host the Silverlight app and click Properties
  3. Click the HTTP Headers tab and select File Types in the MIME Map section
  4. Click New Type and add the following 3 MIME types for Silverlight
    Extension Content Type
    .xap application/x-silverlight-app
    .xaml application/xaml+xml