Multiple file attachments is a supported feature on all account levels. It appears that you may be in Page View. This view is for seeing how the form would appear as a PDF file (where attachments are not supported). In the View menu select Web View. This should then allow you to select the multiple file checkbox in the Field Properties dialog.
Thank you for your answer.
Your answer "web view" is helpfull for me.
On Japanese screen, the explanations are located wrong place.