- you can't upload multiple roms at the same time using drag'n'drop, but you *can* if you click the relevant zone (opening your system's file selection window).
Browse to you local ROMs dir, select the first ROM, go down to the last ROM, then press "shift" while selecting it.
All the ROMs should be selected. Confirm by clicking the upload button.
I had no issue uploading my ~50 snes & ~50 mame ROMs this way (my desktop box is linux btw, but this should work from a windows desktop too i guess).

- upload will be significantly faster if you exit from the menu while uploading (Esc key).
Unsure why, probably cause the menu "eats" something like 20% CPU just being there.