I think with the latest Imagemagick from brew (6.9.4-7) that pdf thumnails no longer get made; the attachment is there but the message "Cannot create thumbnail, please check ImageMagick installation" apears instead of the thumbnails.  I couldn't make out from elogd -v 3 why they failed, but I couldn't make out the exact convert command that was failing.  This is on MacOS 10.11.5 and I think the updated version ofImagemagick was the last thing that changed before it failed.