Microsoft Excel Open XML Document
Microsoft introduced the open XML standard to Microsoft Office in 2007 after a push from business users who saw the value in an open file format for transferring data between applications and other areas of working with the files. Support can also be added to earlier versions of Excel with Open XML component support.
MIME type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
Learn more about XLSX files:
Visit the Microsoft Excel webpage.
Wikipedia has an entry on Office open XML.
Search for answers to XLSX-related questions at IT Knowledge Exchange.