Just thinking out loud —
If we wanted to support resumable uploads in C2S API, then — we probably need some URL to upload the file chunks to.
…
When a user POST to their own outbox, the HTTP "201 Created" response will have a "Location" header that provides a URL.
Maybe that could be used as the upload URL.
Or, maybe the JSON-LD document at that URL might contain a URL under the "object" field that could be used as the upload URL.
Other options too