Function decrypt_file 

pub fn decrypt_file(
    ciphertext: &[u8],
    meta: &EncryptedFileMeta,
    room_crypto: &mut RoomCrypto,
    sender_fingerprint: &str,
) -> Result<Vec<u8>>

Inverse of encrypt_file. The caller supplies the sender’s fingerprint so we know which inbound Megolm session to use.