Programmatically upload/download documents to/from Appian

Setup:
- Appian (Cloud),
- A web application which can access Appian with service user account (i.e. integration user account)

Question: A web application user (not part of Appian) is required to upload/download documents to/from Appian document repository. How can this be achieved?

Couple of options I know are - Create WebAPI and send/receive documents in Base64 in HTTP request/response. But the documents in question could be very big (e.g. audio/video files). This is only good for small documents. Second option could be create a servlet plugin which can receive/send documents, even big documents. Does Appian cloud allow to deploy custom Servlet Plugins? What are the other options?

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