On recent Android versions, "/storage/emulated/0" is the canonical name for the older "/mnt/sdcard".
In Aqua, if you just tap an attachment to view -- it won't be moved to the downloads folder.
If you wish an attachment to be detached from Aqua and be placed into the download folder -- long press it, choose "Save" from the per-attachment "three dots" menu, or use "Save all" at the bottom of the attachment list.
Thanks!
I think the older /mnt/sdcard version belongs to a version before my device. I can only recall seeing /storage/emulated/0/
What I find curious is that while I had the temporary fault, an image attachment did not have a thumbnail (I did not realise this was a fault until the problem had vanished), and I could not just view the attachment either. Instead I got the first error message I reported. It seems that whatever made it impossible to save to download also made it impossible to view*. I tried several other emails with typically PDF or jpg attachments, each of which normally offer me a choice of apps for viewing. Error message instead.
*I assume that in order to view an attachment it has to be saved somewhere temporarily, in order to detach it and pass it to the display app. Clearly this did not work either.
I am hoping this is a transient problem and won't recur!
I now have updated to 1.5.0.22.