Where does Nautilus get its file type description

I've created and registered some shared-mime-info files for custom file types. For example, in

~/.local/share/mime/packages

I've added files like

x-myokit.xml

containing a mime type description, for example

<mime-info xmlns='http://ift.tt/14v9Kt2'>
  <mime-type type="text/x-myokit">
    <comment xml:lang="en">Myokit model definition</comment>
    <glob pattern="*.mmt"/>
  </mime-type>
</mime-info>

Nautilus now recognises my files, but still displays the file type as "Unknown". How do I make it show something more useful in this column?