The pipeline workstream's MediaPlayer takes artifactId, kind, name, poster and size, without a MIME type; the card and the stand-in now use exactly that, so its file replaces the stand-in without other changes. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01MRtGG9ARfqqhma3t85Ro2u