What to do when the incoming download is an older revision than something already in library.

• "block": refuse the download outright before any bytes transfer, surfacing which in-library archive is newer as the failure reason.
• "warn_then_conflict_menu": hold the queue item for explicit user confirmation before starting the transfer; afterwards the existing filename-collision conflict menu still applies if (and only if) the filename actually collides.
• "silent_then_conflict_menu": no upfront prompt, the download just proceeds; same filename-collision-gated conflict menu afterwards.