The standard file field for PloneFormGen allows only adding one file. Adding just a new file field with the option multiple can solve the problem, but we think that the usability of that option is not good. Is not obvious for a lot of people to push Ctrl or Shift to add more than one files. So to address this usability feature, we have added the jquery.filer file selection widget, that shows in a very nice interface the added files with an option to delete them or adding new ones without much effort.
Add this product to your buildout file and rerun the buildout.
Go to Site Setup -> Add ons and install it
A new field will show in your PloneFormGen forms called MultiFileField.