Hi Larry, your post 121 above seems to address deprecation of the Mobile App. It's not clear to me how this is relevant to my issue (No File Attachment button in Conversations).
That's my whole point. That "Attach Files" tab (in your red circle) does not exist for me in CONVERSATIONS.
The "Attach Files" tab does exist, however, in normal threads.
I just checked 3 different devices and 2 different operating systems. The attach files show. Do you see the boxes you can check that allow anyone in the conversation to invite others and/or lock conversation (no responses will be allowed)?