Empty effective whitelist is rejected instead of falling back to all types; the whitelist is matched per extension against core's keys (mp3|m4a|m4b); a file moved into uploads is deleted when the attachment cannot be created; the import request resolves IPv4 only; empty temp name guard for URL imports. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7