Replaces the S-10a stub button with a labelled file input (accept application/pdf); the component base64-encodes the chosen file client-side and posts it (with its name and type) keyed by the reference, confirming on success and surfacing a retryable failure. The upload button stays disabled until a file is chosen. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>