The reason I allow multi-selection is so that you can select both the csv you want to use and the PDF to use with that csv. This is to handle scenarios where the csv and pdf do not have the same name for some reason. This also means that on Windows and iOS, you can select both files at once instead of selecting one, then having to select the other (as the Windows and iOS versions of MobileSheets won't have permission to access a file the user didn't explicitly pick). So it's better for me to be consistent across operating systems in my opinion.
Mike
Mike