_download_slack_file and _download_slack_file_bytes fetched Slack-supplied URLs with the bot token attached and follow_redirects=True, but without the is_safe_url pre-flight check or per-redirect guard that the outbound send_image path already uses. A URL that resolves to (or 3xx-redirects into) a private/internal address could reach internal services and leak the bot token (CWE-918). Add the same pre-flight + _ssrf_redirect_guard hook to both inbound sibling paths.
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