The following video and article explain the 3 different ways that attachments can be added to a SignRequest:
Option 1. The Sender clicks "Add Attachment" when preparing the document.
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Option 2. The Sender can "Require Attachment" from the signer. (In this case, the signer cannot "finalize" the document until the attachment has also been uploaded.
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Option 3. The Signer has the option of uploading an attachment, even if it was not "required" by the sender.
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