_endStream(callback = () => { }) {
    if (this._dest) {
      this._stream.unpipe(this._dest);
      this._dest.end(() => {
        this._cleanupStream(this._dest);
        callback();
      });
    } else {
      callback(); // eslint-disable-line callback-return
    }
  }
Unpipe from the stream that has been marked as full and end it so it
flushes to disk.

@param {function} callback - Callback for when the current file has closed.
@private