You can make it mandatory for the signer to add an attachment. If no attachment is added the document cannot be finalized.
Note: SignRequest cannot check what type of document is added or scan for its validity.
To require an attachment during signing, follow the below steps:
1. Add a document
2. Add a contact (email address)
3. Click the icon to the right of the contact's email address
4. Enter the name of the attachment(s) that you require the signers to add.