You can make it mandatory for the signer to add an attachment. If no attachment is added the document can not be finalised.
Note: SignRequest can not check what type of document is added or scan for its validity.
To require an attachment during signing follow the below steps:
1. After adding a document go to the 'Prepare window'.
2. Open the dropdown as shown below.
3. Add the name of the attachment that the signer needs to upload.