Append from Public slate upload?

How can I append data in a dataset from a Public slate ? How and what should be configured so that the buffer dataset doesn’t get “snapshots” ?
https://www.palantir.com/docs/foundry/slate/public-applications-data-upload

Alternatively, is it possible to directly upload in a mediaset ?

The Secure Upload functionality only supports what is specified in the linked docs. Slate queries only support JSON via HttpJSON datasources.