As a consultant I get many different costumers and the most common way for me to get information and material is in small attachments, one at a time, spread out through the whole project. I don’t think it’s possible to get every new customer to use some kind of FTP or anything else that’s more complicated than their already tried and true way of attaching files in an email.
The point would be to automatically store all attachments in some form of group. Either a new view in Outlook/Gmail, or just automatically saving all attachments to a folder. It could prompt if it doesn’t know what project the attachments belongs to.