As discussed internally: Instead of merging the thumbnails we should do the following:
On the attachments per screen page ask the user whether we should generate thumbnails. Also add a notice about changing settings and rerunning the thumbnail creation after the merge. Default is 'Yes'
After the attachment has been copied (after_insert) we check that setting and create the thumbnail there.
As discussed internally: Instead of merging the thumbnails we should do the following:
after_insert) we check that setting and create the thumbnail there.