While hosting workflow service files (XAMLX) in IIS 8, you may receive the following error:

HTTP Error 404.3 – Not Found

The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map.

Enabling WCF HTTP Activation resolved the issue for me.

• Open the Windows Features screen
• drill down to .NET Framework 4.5  Advanced Services-> WCF Services
• Check HTTP Activation

After this change both .xamlx and .svc files appeared in the IIS Handler Mappings section, and the 404.3 errors stopped.