Additional attributes for custom document type not showing in Finder Get Info

Question

My Spotlight plugin imports a couple of attributes for a custom document format.
Searching for them works just fine but I just can't get them displayed in the Finder's Get Info dialog.

Here's the schema.xml (parses fine with mdcheckschema):

<?xml version="1.0" encoding="UTF-8"?>
<schema version="1.0" xmlns="http://www.apple.com/metadata"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://www.apple.com/metadata
   file:///System/Library/Frameworks/CoreServices.framework/Frameworks/Metadata.framework/Resources/MetadataSchema.xsd">

   <types>
      <type name="com.mycompany.custom">
         <note>
            The keys that this metadata importer handles.
         </note>
         <allattrs>
            kMDItemVersion
            kMDItemLayerNames
            kMDItemDescription
            kMDItemAuthors
         </allattrs>
         <displayattrs>
            kMDItemDescription
            kMDItemAuthors
            kMDItemVersion
            kMDItemLayerNames
         </displayattrs>
      </type>
   </types>

</schema>

Are there tools to debug whether a schema is used at all..?

Also looking at the importers in /System/Library/Spotlight/ many of them do not include a schema.xml at all (nor does the default Xcode 4.x Spotlight Importer project but that could be a bug) so starting to wonder..