Dimensions CM should tell CM Desktop users that Add Item does not support multi-file uploads.
(I haven't checked if web client tools has the same behavior - it might)
The replacement message I propose could be similar to the following:
"Please select a single file instead of a folder, or use for multiple files"
The behavior cited is for CM 14.5.3 (Build 9.245)
Little-known to end-users is the KB article:
at this link:
which states that in order for multi-file uploads to work for Add Item operations,
the end-user must have a UNIX-derived, 3rd party binaries on their machine.
This bypasses CM revision history capabilities and also requires folks who download it
to likewise have the .tar binaries configured with their CM Desktop client instance.
This is far too technical for most users and outright gets around revision history,
which is one of the main reasons to use Dimensions CM.
That is why I believe that unless the user's CM Desktop client is configured for
that 3rd party .tar configuration, the CM Desktop client 'should' unequivocally
tell users to select a file if a folder is selected.