Export datatable to excel file

Hello gurus!

I use the following code (see bottom of post) to create a native xls file, with Microsoft Office 2007 installed. However, when I open the file there's always a warning telling me:

"The file you are trying to open, 'file.xls' is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening this file. Do you want to open the file now?"

When clicking "Yes" the file opens just fine.

My question is: How can I get rid of this warning? It is obviously not corrupted, and the datatable contains no special characters. I don't see how the format is related to "trusted source", whatever that means. Can someone please explain?

Code:

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Protected Sub ExportAsExcelFile(ByVal dtExport As DataTable)

        Response.Clear()
        Response.ContentType = "application/vnd.ms-excel"

        Dim objStringWriter As New System.IO.StringWriter
        Dim objHtmlTextWriter As New System.Web.UI.HtmlTextWriter(objStringWriter)

        Dim objDatagrid As New DataGrid
        objDatagrid.DataSource = dtExport
        objDatagrid.DataBind()

        objDatagrid.RenderControl(objHtmlTextWriter)

        Response.Write(objStringWriter.ToString())
        Response.End()

End Sub

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Nothing wrong in your source code. It's only because you have Office 2007 installed. Did you ever try on another computer having different verson of Office? Webpage only exports 97-2003 excel format, you would get the warning if you use 2007 to open it.