http://msdn.microsoft.com/en-us/library ... 85%29.aspx
File type and URI associations model
Platforms
Clients - Windows 8
Servers - Windows Server 2012
Description
The file type and URI association model has changed in Windows 8. Apps are no longer able to programmatically set themselves as the default handler for a file type or URI. Instead, now the user always controls what the default handler is for a file type or URI scheme.
Manifestation
How this change presents to the user depends upon how the app is designed, for example:
Many apps check to see if they are the default every time they run and, if they are not, they prompt the user to set them as default. However, because apps can no longer accurately query to determine which app is the default handler for a file type or URI scheme, neither of these operations works.
Many apps have a dialog box or menu built in or in their installer that specifies the file types for which the app should serve as the default. However, because apps can no longer programmatically set themselves as the default handler for a file type or URI scheme, this no longer works.