I read this line from HTML5 Boilerplate :
HTML Manifest files now use .appcache extension instead of .manifest, as per http://html5.org/r/5812.
And I saw this from http://html5.org/r/5812
Change the suggested extension for appcache manifests from .manifest to .appcache to avoid clashing with Microsoft's unregistered application/manifest type
Here is my question. What is Microsoft's unregistered application/manifest type and when does it clash?