This is the most popular file type for spreadsheets. It has an open equivalence, the ods format, which is the default format of LibreOffice.
Name : Abreviation of Excel
Mime : application/vnd.ms-excel
Licence : Proprietary
Type : Document Tableur
Free softwares to open xls : LibreOffice Calc
Non free softwares to open xls : Microsoft Excel