HTML5 appcache manifest file extension

Question

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?

Answer 1

The Microsoft technology is proprietary and related to synchronising sites / applications between IIS instances I think.

The "clash" relates to the fact that off-line cache manifest files typically use the .manifest file extension, but require a MIME type of text/cache-manifest to work, whereas the unrelated MS .manifest files use the MIME type you mention, i.e. application/manifest