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Base on my test just now, the feature request from the other thread is not yet implemented. It has low priority unfortunately due to a low number of request.
For now, on the new Dropbox integration wizard, when you leave the root folder name empty, it will create a default folder named "JotForm" and this is where the files go through if you select "No Folder" on the sub-folder.
About the original file name, yes they are retained when you select a sub-folder from the fields.
Here are example setup and how they will end up on Dropbox.
This outputs the following - file names are appended with the submission ID.
Now when you select a field as the sub-folder - name of the files will be retained.
Hope this information helps!