The New Attachment button was not working. It started my day working (I could successfully add attachments) and stopped later in the day (nothing happened if i clicked on the button). I logged out and logged in - still not working.
I tried opening an old receipt (which already had an attachment) to see if i could add more to that existing attachment - I could not. It was also problematic.
So not contradictory - just trying to figure out if posts i had made previously with existing attachments were also affected. They were. I could not add further attachments.
I tried again after your first post - and it worked.