See UniversalMediaServer/UniversalMediaServer#2124 (comment)

Hi, I've been trying to get additional tags from itunes m4a files. for example album artist which should be aART based on https://help.mp3tag.de/main_tags.html

I added this to config.xml

<library-options>
      <id3>
        <auxdata>
          <add-data tag="aART"/>
        </auxdata>
      </id3>
</library-options>

and import.js

var aartist = obj.aux['aART'];

It would be good to set it by hash(machine-id, ip, port) or something similar