When reading the xls file as attachment, some cells cannot be read. Code as following:

var wb = Workbook.Load('testfile.xls');

var ws = wb.Worksheets[0];

Debug.WriteLine(ws.Rows[1].Cells[0].Value);    // This line cause the app hang up

Debug.WriteLine(ws.Rows[1].Cells[1].Value);    // But other columns can be read fine