File description

Supported Microsoft formats are Spreadsheets (.xls) and Excel Workbooks (.xlsx). 
.xlsx is the default Excel 2007 and later workbook format.

Good to know

Documentation

Official Microsoft Developer Network website: http://msdn.microsoft.com/en-us/library/gg615597(v=office.14).aspx
Wikipedia: http://en.wikipedia.org/wiki/Microsoft_Excel#File_formats

Sample file

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See also: