In MP version 2.9.0.1 and Form Builder 2.9 if you allow uploads the form will not submit. There's no error in the log and I've tried various settings but nothing will allow the form to submit if attachments are allowed. It won't let you to "next" to the second page with the uploads, the next button just refreshes the page and clears the form. If you are logged in as an admin it will work but an anonymous user can't progress to the upload page. I tried it on a fresh form and in multiple installs.