A file with the XLSX file extension is a Microsoft Excel Open XML Format Spreadsheet file. It's a ZIP-compressed, XML-based spreadsheet file created by Microsoft Excel version 2007 and later.

Doesn't seem to matter the OS or Office version, but it's also only xls/xlsx files. csv's .doc and so forth open without issue.

XLS files are Excel workbook files created in Excel 97 or Excel 2003. The 2007, 2010 and 2013 versions of the program use the XLSX format by default to save workbooks, but can also use the XLS format.

What happens when you want to edit a Microsoft Excel .xlsx file on a Mac, iPhone or iPad? While it’s not quite as easy as opening .docx files in Pages for Mac, it’s not an overly complicated ...