Populated MediaAttachment ready to be passed to
createImetaTagForAttachment (after setting the url field to the
Blossom upload URL).
The sha256 field contains the hex-encoded SHA-256 of the plaintext
file (the x imeta field), used for integrity verification after
decryption.
The nonce field contains the hex-encoded 12-byte nonce used during
encryption.
The encrypted blob. Upload this to Blossom and use SHA256(encrypted) as the blob address.
Result of encryptMediaFile.