Read an excel file using Koogra

KoograRead.cs

// This example shows how to read and excel file using the Koogra library (http://sourceforge.net/projects/koogra/)
// Supports .xls and .xlsx example

public static string ReadExcelContent(string filePath)
{
        uint rowNum = 0;
        uint colNum = 0;
        var data = new StringBuilder();
        try
        {
            Net.SourceForge.Koogra.IWorkbook wb;
            string fileExt = Path.GetExtension(filePath);

            if (string.IsNullOrEmpty(fileExt))
            {
                throw new Exception("File extension not found");
            }

            if (fileExt.Equals(".xlsx", StringComparison.OrdinalIgnoreCase))
            {
                wb = Net.SourceForge.Koogra.WorkbookFactory.GetExcel2007Reader(filePath);
            }
            else if(fileExt.Equals(".xls", StringComparison.OrdinalIgnoreCase))
            {
                wb = Net.SourceForge.Koogra.WorkbookFactory.GetExcelBIFFReader(filePath);
            }

            Net.SourceForge.Koogra.IWorksheet ws = wb.Worksheets.GetWorksheetByIndex(0);

            for (uint r = ws.FirstRow; r <= ws.LastRow; ++r)
            {
                
                Net.SourceForge.Koogra.IRow row = ws.Rows.GetRow(r);
                if (row != null)
                {
                    string lineEnding;
                    for (uint colCount = ws.FirstCol; colCount <= ws.LastCol; ++colCount)
                    {
                        string cellData = string.Empty;
                        if (row.GetCell(colCount) != null && row.GetCell(colCount).Value != null)
                        {
                            cellData = row.GetCell(colCount).Value.ToString();
                        }

                        if (colCount == ws.LastCol)
                            lineEnding = Environment.NewLine;
                        else
                            lineEnding = "\t"; //tab
                        data.Append(string.Concat(cellData, lineEnding));
                    }
                }
            }

        }
        catch (Exception ex)
        {
            Exception exception = ex;
            exception.Source = string.Format("Error occured on row {0} col {1}", rowNum, colNum);
            throw ex;
        }

        return data.ToString();
}

hello ,, this code is working fine for Excel sheets with extension of ".xlsx" i.e latest but not reading the sheets for those with extension ".xls" ... Can you plz help me out in this.
Thanks in advance... :)