The demo seeded `cover.png` with `new Uint8Array(size)` — empty
bytes. `URL.createObjectURL` then handed a blob the browser couldn't
decode as PNG, so the preview rendered the user-agent broken-image
icon next to the row.
Fix: when the demo file type starts with `image/`, prepend a valid
1x1 transparent PNG header before the size-padding bytes. The
browser decodes a 1px image cleanly and the preview slot stays
empty-looking (which is the intended visual) instead of showing a
broken-image affordance. atob lives inside the `typeof File`
runtime check so SSR doesn't choke on it.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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