title: string|null, // ex. "Arch Nemesis: Mail Carrier" or "Parcel #5055"
description: string|null, // free-form description
media: string|null, // URL to associated media, preferably to decentralized, content-addressed storage
media_hash: string|null, // Base64-encoded sha256 hash of content referenced by the `media` field. Required if `media` is included.
copies: number|null, // number of copies of this set of metadata in existence when token was minted.
issued_at: string|null, // When token was issued or minted, Unix epoch in milliseconds
expires_at: string|null, // When token expires, Unix epoch in milliseconds
starts_at: string|null, // When token starts being valid, Unix epoch in milliseconds
updated_at: string|null, // When token was last updated, Unix epoch in milliseconds
extra: string|null, // anything extra the NFT wants to store on-chain. Can be stringified JSON.
reference: string|null, // URL to an off-chain JSON file with more info.
reference_hash: string|null // Base64-encoded sha256 hash of JSON from reference field. Required if `reference` is included.