EXport to excel Issue

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C#

.NET

string filename = "FY15 Input Collection Template_" + ddlcity.SelectedItem.Text+".xls" ;
Response.AddHeader("content-disposition", "attachment;filename=" + filename);
Response.Charset = "UTF-8";
Response.ContentType = "application/vnd.ms-excel";
StringWriter tw = new StringWriter();
HtmlTextWriter hw = new HtmlTextWriter(tw);
Microsoft.Office.Interop.Excel.ApplicationClass _ExcelApp = new Microsoft.Office.Interop.Excel.ApplicationClass();
_ExcelApp.DisplayAlerts = false;
tbcity.RenderControl(hw);
tbannualpercent.RenderControl(hw);
tblasm.RenderControl(hw);
tbtwotable.RenderControl(hw);
tbsectb.RenderControl(hw);
tbthreetb.RenderControl(hw);
tbthirdtb.RenderControl(hw);
tbfourtb.RenderControl(hw);
tbfrtable.RenderControl(hw);
this.EnableViewState = false;
Response.Write(tw.ToString());
Response.Flush();
Response.End();


when the file is downloaded and when we tring to open the excel file its showing pop-up msg "FILE EXTENSIONS WERE CORRUPTED DO YOU WANT TO OPEN ANY WAy" how can i suppress that pop up message any help please......