Select the HTTP Headers tab and then click the MIME Types... button
Click the New button
Enter .xap for the Extensions and application/x-silverlight-app for the MIME type
Click OK, then OK, then Apply, then OK.
Setting .xap MIME Type for Silverlight
http://www.adefwebserver.com/dotnetnukehelp/misc/Silverlight/SettingMimeType.html
Windows 2003:
In IIS Management, click on the root node under Internet Information Services
Select the HTTP Headers tab and then click the MIME Types... button
Click the New button
Enter .xap for the Extensions and application/x-silverlight-app for the MIME type
Click OK, then OK, then Apply, then OK.
Select the HTTP Headers tab and then click the MIME Types... button
Click the New button
Enter .xap for the Extensions and application/x-silverlight-app for the MIME type
Click OK, then OK, then Apply, then OK.