The ability to upload documents to an Act-On form is not currently possible. But don't let that limit your use of Act-On forms for this purpose! Act-On forms are super key to your marketing automation success because they help convert anonymous individuals to known individuals.
The best way to workaround this upload restriction is via the form Confirmation Email. If you need to require documents as part of your form workflow, you can request these via a form Confirmation Email that has clearly outlined steps on what file type you require, allowing your form submitter the ability to send over the document as an attachment.
But what if you only need some people to send documents as a part of your form workflow? This can be accomplished through Form Submission List segmentation and an Automated Program.
First up - find your Form Submission List that is recording submissions for the form in question. Create a segment under this list using profile fields to segment out the particular response option that would require a follow up email asking for documents.
Next up, create an Automated Program using your Form Submission List as the source list, and set it to add new contacts immediately:
In the Program Messages tab, add a general form confirmation email and a form confirmation email that requests the necessary attached files. In the Lists & Segments tab, add the "Follow up requesting documents" segment.
Next, head to Program Flow and add a conditional Send Message step:
The end result is sending follow up emails based on their form response: one for general confirmation purposes, and one that sends an email asking for the necessary attachments.
Hopefully this option provide some workarounds to help you achieve the form response needs for your organization! Happy marketing :)