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I’ve found that I get this error every time I upload a Silverlight application to a new webserver. IIS does not have the correct settings by default.

The Problem: You ftp a Silverlight App to an IIS website, go to the website in a browser, and get the following error:

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

The Cause: The Silverlight Mime types (.xaml, .xap and .xbap) are not listed in that websites registered mime types, therefore IIS will not send those files to the browser.

The Solution: Open IIS on the server, go to HTTP headers, click on Mime Types, click “Add New” and add the following:

Extension – Mime Type
.xaml – application/xaml+xml
.xap – application/x-silverlight-app
.xbap – application/x-ms-xbap

Hit Apply and your application will load!

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