Hmm, yeah you need "Open File Result" action actually.
Just read what that action says in description and you
should be able to upload all files at once >>>
" Uploading file is performed in two steps: setting next open file dialog result with this action and clicking on element which triggers file upload, like "Upload" button.
You must always call this action before starting upload, otherwise BAS won't know which file it need to send to browser.
Use resource with type FilesFromDirectory to upload all files from folder and distribute them across threads."