Use this for files above ~3.5 MB, which is where the single-shot POST /users/file/upload route stops being reliable behind Lambda. Returns a sessionId and the server-preferred chunkSize (4 MB) — always split on the returned value rather than hardcoding it. The client then uploads each part via /chunk (1-indexed partNumber) and finalises via /complete, which returns the File.id to attach with POST /users/{id}/data.