You can reply to an existing discussion thread in the list by clicking the detail button.
Then the selected discussion thread with existing data will appear.
You can reply thread and attach the file.
Click send button to reply the thread and you reply will appear in the thread.
You also can edit the member of discussion thread from here.
If you are not the thread creator, you can't edit the thread. forbidden
You can check all file attached in the thread and download it.