EnvelopeAttachments Resource
Envelope attachments are files that are associated an envelope. They are not visible to senders or to recipients and are not converted to PDF or any other format. These are files that your application uses to store information.
Envelope attachments are available only through the API.
It’s easy to confuse envelope attachments, which are developer-only files associated with an envelope, with signer attachments.
To learn about the different types of attachments, see Attachments in the concept guide.