Release 2.3.9.143 on C7
File manager gives the impression it can handle upload of multiple files but in fact it only takes one at a time.
I selected 3 files from my local file browser, and dragged them to the place it said to drag here on HE File Manager. The tool showed "3" and then "+" when over the field. None of them were actually added to the screen. Repeated the attempt a few times.
Then selected one file at a time from my local files and dragged each one to the Drag Here field. Each one was added by name and tagged as "Pending." Finally I clicked on Upload and a message told me the files were uploaded and all of them had gone from the Drag Here field. In fact only one file had been uploaded. I would suggest that if HE can only operate on one file at a time that is made clear via the UI.