Tried on my laptop using Opera and Chrome, getting the same problem there.
I'll upload a test image here as an example:

Edit: Seems to work now. Huh.
It showed up as the grey fakepath image in preview though, maybe that's intentional, I figured it might be the first time I had this problem and posted anyways, only to get an invalid attachment. Maybe due to file size? I don't know.
I'll try and post a proper attachment and see what happens then, might be able to mark this as human error

Edit2:
Yeah, seems like I might have tried uploading something over the size limit the first time and from then on automatically assumed that the preview placeholder meant something was wrong.