Review findings on #109139 (the "aeskey half is already on main" claim was
wrong): `_decrypt_file_bytes` restored base64 padding but the key was never
percent-decoded, so `%2F`/`%3D` keys failed to decrypt and `_cache_media`
returned None. And `_store_media` received the CDN's `application/octet-stream`
as the image MIME, which the gateway image classifier rejects even with the
corrected `.png` name.
Decode the key once at the payload boundary; for images forward the response
MIME only when it is `image/*`, otherwise derive it from the resolved
extension. Covered by one end-to-end test through `_cache_media` (red on the
previous head).
5fd8f02e3c