In Generated module HasAttachment is used. Multi file-upload is out of the box as attachments.
HasAttachment trait is on Entity in module.
If You want to add file field directly to create/edit form. Then You need to add file field to form (located in ../Htp/Forms/…Your Form).
User module in Platform is example of file directly in Form.
Please check /Modules/Platform/User module – check form and user controller.