def set_encrypted_uploadable(u):
    """Provide a source of encrypted upload data. 'u' must implement
        IEncryptedUploadable.

        When this is called, the IEncryptedUploadable will be queried for its
        length and the storage_index that should be used.

        This returns a Deferred that fires with this Encoder instance.

        This must be performed before start() can be called.
        """
