public class ReportService : Service
{
    public IReportRepository Repository {get; set;}

    public object Get(GetInvoiceReport request)
    {
        var invoices = Repository.GetInvoices();

        ExcelReport report = new ExcelReport();
        byte[] bytes = report.Generate(invoices);

        return new FileResult(bytes);
    }
}

public class FileResult : IHasOptions, IStreamWriter, IDisposable
{
    private readonly Stream _responseStream;
    public IDictionary<string, string> Options { get; private set; }

    public BinaryFileResult(byte[] data)
    {
        _responseStream = new MemoryStream(data);

        Options = new Dictionary<string, string>
        {
            {"Content-Type", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"},
            {"Content-Disposition", "attachment; filename="InvoiceFile.xlsx";"}
        };
    }

    public void WriteTo(Stream responseStream)
    {
        if (_responseStream == null)
            return;

        using (_responseStream)
        {
            _responseStream.WriteTo(responseStream);
            responseStream.Flush();
        }
    }

    public void Dispose()
    {
        _responseStream.Close();
        _responseStream.Dispose();
    }
}

[Test]
public void TestInvoiceReport()
{
    var client = new JsonServiceClient("http://localhost/report/");

    var authResponse = client.Send(new Authenticate
                {
                    provider = CredentialsAuthProvider.Name,
                    UserName = "[User Name]",
                    Password = "[Password]"
                });

    var requestDTO = new GetInvoiceReport();

    var ret = client.Get<FileResult>(requestDTO);

    Assert.IsTrue(ret != null);
}