private ClosedXML.Excel.XLWorkbook GenerateClosedXMLWorkbook()
{
    var wb = new ClosedXML.Excel.XLWorkbook();
    var ws = wb.AddWorksheet();
    ws.FirstCell().SetValue("Hello world!");
    ws.FirstCell().CellBelow().FormulaA1 = "RAND()";
    return wb;
}

public ActionResult Download()
{
    using (var wb = GenerateClosedXMLWorkbook())
    {
        // Add ClosedXML.Extensions in your using declarations
        
        return wb.Deliver("generatedfile.xlsx");
        
        // or specify the content type:
        return wb.Deliver("generatedFile.xlsx", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet");
    }
}