added link for Open XML mime types

Author: gbinal

schema.md

@@ -211,7 +211,7 @@ Further Metadata Field Guidance (alphabetical by JSON field)
 **Cardinality** | (0,1)
 **Required** | Yes, if the file is available for public download.
 **Accepted Values** | String
-**Usage Notes** | This must describe the exact files available at **accessURL** using [MIME Types](http://en.wikipedia.org/wiki/Internet_media_type).  
+**Usage Notes** | This must describe the exact files available at **accessURL** using [MIME Types](http://en.wikipedia.org/wiki/Internet_media_type).  _[Also note [Office Open XML MIME types](http://blogs.msdn.com/b/vsofficedeveloper/archive/2008/05/08/office-2007-open-xml-mime-types.aspx)]_
 **Example** | `{"format":"application/json"}`
 
 {: .table .table-striped}