[ Thanks to Dmitri for
this link. ]
“Choose Settings → Configure digiKam and switch to the
MIME Types section. To exclude, for example, the PPM format, enter
-ppm in the Image Files field. You can add multiple formats using a
space as separator. Excluding specific formats from digiKam doesn’t
actually delete them, but simply hides them from digiKam and
removes all metadata from the digiKam databases. So be careful when
using exclusion filters if you don’t want to lose tags, ratings,
and other metadata.”