How to resolve the problem "Could not download sliverlight application"

If your code uses ".xap" and your server does not register the MIME type in IIS, you will get this error.

To resolve it:

1. Simply open the IIS7.0
2. Click MIME in Feature window
3. Click Add button on right hand side.
4. For the type in MIME input following:
Custom Header Name: .xap
Custom Header Value: application/x-silverlight-app