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[ServiceContract]
public interface ISearch
{

    [OperationContract]
    [WebGet(ResponseFormat = WebMessageFormat.Json)]
    byte[] DownloadFile2();
}                    

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    public byte[] DownloadFile2()
    {
        if (WebOperationContext.Current == null) throw new Exception("weboperationcontext not set");

        ExcelPackage exl = new ExcelPackage();

        ExcelWorksheet wsh = exl.Workbook.Worksheets.Add("tabelle1");

        wsh.Cells[1, 1].Value = "company name";
        wsh.Cells[1, 2].Value = "address";
        wsh.Cells[1, 3].Value = "status";

        WebOperationContext.Current.OutgoingResponse.ContentType = "application/octet-stream";
        WebOperationContext.Current.OutgoingResponse.Headers.Add("content-disposition", "inline; filename=tes.xlsx");

        return exl.GetAsByteArray();
    }