Do you use the regular Aras batch loader? As far as I know, uploading files with the batch loader was possible in the past, but is no longer possible in newer Aras version like 12.
Edit: I think this one was caused by security restrictions in modern browsers (so Aras it not to blame). In the past it was possible to upload files from the clients PC just via a path. This of course is a potential risk. So right now when you want to upload files this way, you need to build an own solution with file picker.