With FileStreamResult, how is the MemoryStream closed?

The FileStreamResult will do that for you. When in doubt always check the code, because the code never lies and since ASP.NET MVC is open source it’s even more easy to view the code.

A quick search on Google for FileStreamResult.cs lets you verify that in the WriteFile method the stream is correctly disposed using the using statement. (no pun intended)

protected override void WriteFile(HttpResponseBase response) {
    // grab chunks of data and write to the output stream
    Stream outputStream = response.OutputStream;
    using (FileStream) {
        byte[] buffer = new byte[_bufferSize];

        while (true) {
            int bytesRead = FileStream.Read(buffer, 0, _bufferSize);
            if (bytesRead == 0) {
                // no more data
                break;
            }

            outputStream.Write(buffer, 0, bytesRead);
        }
    }
}