File for Premium members only — sign in and upgrade
Mint/stream/preview refused a Premium-only file for a guest (or unsigned viewer). Full text asks them to sign in and upgrade. Signed-in non‑Premium members more often see the shorter “Upgrade your account…” sibling (Phase 17).
Why this happens
Same isPremium flag as the landing-page wall, enforced by assertPremiumFileDownloadAccess. Owner always passes. Distinct from site-wide “downloads aren't available as a guest” (tier Downloads enabled).
Diagnose and fix
1. Visitor: create an account, buy Premium / Premium Pro, retry.
2. Uploader: turn off Premium on the file if guests should download it.
3. Admin: check Files premium by default so new uploads are not all locked.
4. Confirm Premium plans are purchasable.