ปัญหาการ Download excel file จากการ generate โดยใช้ EPPLUS บน C# ASP.NET .NET FrameWork 4.6.1

$.ajax({
            type: "POST",
            crossOrigin: true,
            url: "/api/Transaction/ExportExcel",
            headers: {
                Accept: "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
            },
            data: {
                "Date_1": dateTime[0],
                "Date_2": dateTime[2],
                "PaymentType": (paymentType === "99" ? '' : paymentType),
                "Status": (paymentStatus === "99" ? '' : paymentStatus),
                "Outtrade": refNo,
                "Merchant": (merchantName == "00" ? '' : merchantName),
                "TerminalID": terminalId
            },
            success: function (result) {

                if (dateTime[0] == dateTime[2]) {
                    filename = "Transaction_Detail_" + dateTime[0].replace(/\//g, '-') + ".xlsx";
                }
                else {
                    filename = "Transaction_Detail_" + dateTime[0].replace(/\//g, '-') + " - " + dateTime[2].replace(/\//g, '-') + ".xlsx";
                }

                var uri = 'data:application/vnd.openxmlformats-officedocument.spreadsheetml.sheet,' + result;
                var link = document.createElement("a");
                link.href = uri;
                link.style = "visibility:hidden";
                link.download = filename;
                document.body.appendChild(link);
                link.click();
                document.body.removeChild(link); 
            },
            error: function (jqXHR, exception) {
                console.log("Post error");
                $('#btnApply').removeAttr("disabled");
                $('#btnExportExcel').removeAttr("disabled");
                getErrorMessage(jqXHR, exception);
            }
        });
        return deferred.promise();
    });

public class TransactionController : ApiController
{
        [AllowAnonymous]
        [HttpPost]
        [Route("api/Transaction/ExportExcel")]
        public HttpResponseMessage ExportExcel(TransactionSearchReq req)
        {
            TransactionMgr transaction = new TransactionMgr();
            List<TransactionRecordRes> transactionList = new List<TransactionRecordRes>();
            HttpResponseMessage response = Request.CreateResponse(HttpStatusCode.OK);
            //MediaTypeHeaderValue mediaType = MediaTypeHeaderValue.Parse("application/octet-stream");
            MediaTypeHeaderValue mediaType = MediaTypeHeaderValue.Parse("application/vnd.openxmlformats-officedocument.spreadsheetml.sheet");
            //response.Content = new StringContent("hello", Encoding.Unicode);
            byte[] fileContents;
            string fileName;
            try
            {
                // Get transaction records
                transactionList = transaction.GetSearchTransactionRecord(req);
                if (transactionList.Count() > 0)
                {
                    ExcelPackage.LicenseContext = LicenseContext.NonCommercial;
                    //var contentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
                    using (var package = new ExcelPackage())
                    {
                        var worksheet = package.Workbook.Worksheets.Add("Sheet1");

                        // Put whatever you want here in the sheet
                        worksheet.Cells["A1"].Value = "Date Time";
                        worksheet.Cells["A1"].Style.Font.Size = 12;
                        worksheet.Cells["A1"].Style.Font.Bold = true;
                        worksheet.Cells["A1"].Style.Border.Top.Style = ExcelBorderStyle.Hair;

                        worksheet.Cells["B1"].Value = "Merchant Name";
                        worksheet.Cells["B1"].Style.Font.Size = 12;
                        worksheet.Cells["B1"].Style.Font.Bold = true;
                        worksheet.Cells["B1"].Style.Border.Top.Style = ExcelBorderStyle.Hair;

                        worksheet.Cells["C1"].Value = "Terminal ID";
                        worksheet.Cells["C1"].Style.Font.Size = 12;
                        worksheet.Cells["C1"].Style.Font.Bold = true;
                        worksheet.Cells["C1"].Style.Border.Top.Style = ExcelBorderStyle.Hair;

                        worksheet.Cells["D1"].Value = "Payment Type";
                        worksheet.Cells["D1"].Style.Font.Size = 12;
                        worksheet.Cells["D1"].Style.Font.Bold = true;
                        worksheet.Cells["D1"].Style.Border.Top.Style = ExcelBorderStyle.Hair;

                        worksheet.Cells["E1"].Value = "Outtrade";
                        worksheet.Cells["E1"].Style.Font.Size = 12;
                        worksheet.Cells["E1"].Style.Font.Bold = true;
                        worksheet.Cells["E1"].Style.Border.Top.Style = ExcelBorderStyle.Hair;

                        worksheet.Cells["F1"].Value = "Amount";
                        worksheet.Cells["F1"].Style.Font.Size = 12;
                        worksheet.Cells["F1"].Style.Font.Bold = true;
                        worksheet.Cells["F1"].Style.Border.Top.Style = ExcelBorderStyle.Hair;

                        worksheet.Cells["G1"].Value = "Status";
                        worksheet.Cells["G1"].Style.Font.Size = 12;
                        worksheet.Cells["G1"].Style.Font.Bold = true;
                        worksheet.Cells["G1"].Style.Border.Top.Style = ExcelBorderStyle.Hair;

                        for (int i = 0; i < transactionList.Count(); i++)
                        {
                            worksheet.Cells[i + 2, 1].Value = transactionList[i].DateTime;
                            worksheet.Cells[i + 2, 2].Value = transactionList[i].Merchant;
                            worksheet.Cells[i + 2, 3].Value = transactionList[i].TerminalID;
                            worksheet.Cells[i + 2, 4].Value = transactionList[i].PaymentType;
                            worksheet.Cells[i + 2, 5].Value = transactionList[i].Outtrade;
                            worksheet.Cells[i + 2, 6].Value = transactionList[i].Amount;
                            worksheet.Cells[i + 2, 7].Value = transactionList[i].Status;
                        }

                        // So many things you can try but you got the idea.
                        // Finally when you're done, export it to byte array.
                        fileContents = package.GetAsByteArray();
                    }

                    if (fileContents == null || fileContents.Length == 0)
                    {
                        response = Request.CreateResponse(HttpStatusCode.InternalServerError);
                        response.Content = new StringContent("Export file error.", Encoding.Unicode);
                        //return NotFound();
                        return response;
                    }
                    else
                    {
                        if (string.Compare(req.Date_1, req.Date_2) == 0)
                        {
                            fileName = string.Format("Transaction_Detail_{0}.xlsx", req.Date_1.Replace('/', '-'));
                        }
                        else
                        {
                            fileName = string.Format("Transaction_Detail_{0} - {1}.xlsx", req.Date_1.Replace('/', '-'), req.Date_2.Replace('/', '-'));
                        }

                        //return File( fileContents, contentType, fileName );
                        MemoryStream memoryStream = new MemoryStream(fileContents);
                        response.Content = new StreamContent(memoryStream);
                        response.Content.Headers.ContentType = mediaType;
                        response.Content.Headers.ContentDisposition =
                            new ContentDispositionHeaderValue("attachment") { FileName = fileName };
                        return response;
                    }
                }
                else
                {
                    response = Request.CreateResponse(HttpStatusCode.InternalServerError);
                    response.Content = new StringContent("Export file error.", Encoding.Unicode);
                    //return NotFound();
                    return response;
                }
            }
            catch (Exception ex)
            {
                response = Request.CreateResponse(HttpStatusCode.InternalServerError);
                response.Content = new StringContent("Export file error.", Encoding.Unicode);
                //return NotFound();
                return response;
            }
        }
}