.xls is the default file extension for spreadsheets created using Microsoft Excel 97-2003. Microsoft spreadsheet solution called MS Excel is one of the best spreadsheet solutions available with desktop publishing capabilities. The .xls files can contain formatted tables, formulas, text, images, tables, graphs, charts, page formatting, and print settings. Microsoft office 2007 onwards the extension was changed to .xlsx Microsoft Excel files use a proprietary format for storing Microsoft Excel documents. This file format is known as the Binary Interchange File Format (BIFF). Data inside all Office Document files are stored in series of fixed-size streams. The location of each fixed-size stream is described by several sector allocation tables. These streams contain meta-data information about the document, such as the author name, subject, and individual sheet names.