Create Upload

A reserved single-request upload.

PUT the file bytes to upload_url with exactly the declared Content-Type and Content-Length (both are part of the signature), then call the complete endpoint with upload_id.

A reserved multipart upload.

Slice the file into part_size chunks (the last part may be short), PUT each slice to its part's upload_url — in parallel if desired — and collect each response's ETag header. Then call the complete endpoint with upload_id, s3_upload_id, and the part/ETag pairs.