always decide on image file extension

instead of choosing jp2

Signed-off-by: Sebastian Schüpbach <sebastian.schuepbach@unibas.ch>
2 jobs for master in 1 minute and 46 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #67443
mbr
test

00:00:32

 
  Publish
passed #67444
mbr
build-latest-image

00:01:14