Initiate a resumable upload

Optional client-computed sha256 of the file bytes (64 lowercase hex chars). When supplied, the platform checks for an existing non-archived sample in the same org with the same hash; on a hit the response carries existing: true and the existing uploadId instead of minting a new signed URL.

Resumable upload URL — PUT slide bytes here. Omitted when existing: true.

True when contentSha256 matched an existing non-archived sample in the same org. In that case uploadUrl is omitted and the byte upload should be skipped.