Actually what I just noticed - is that upon installing the Fossify messages, and opening an attachment, it asked me which app I wanted to use, and I just recently installed Fossify gallery, so when I choose that one, there's no problem, but interesting thing is, even in Fossify messages, if I try to open the attachment in the built-in gallery, it doesn't work - note: this is the black+white gallery app that comes with GOS.