Currently we don't support this. This has been a fairly common request and is on our roadmap for this year.
We appreciate your patience. You'll be seeing us roll out features on a monthly basis going forward - we just released page branching, twitter integration and redirect after submit yesterday.
Sorry that this has taken so long. We've just added support for attachments in our latest release (12/5/2011). You can read more about the feature:
Please try it out and give us your feedback.