NC13 & Dicom viewer 0.06: How to generate preview images?

According to the configuration instructions of the preview app i have extended config.php with one line:

9 => 'OC\\Preview\\DCM',

But after running the preview updater script

sudo -u www-data php occ preview:generate-all

the preview image is greyed out instead of displaying it.

How can i enable previews for Dicom images?