A Microsoft Excel workbook in the Office Open XML format.
| 0x00000000 | 504B0304 | PK·· | 'PK' — it is a ZIP |
Inside the ZIP: '[Content_Types].xml' plus an 'xl/' directory.
A .xlsx is a ZIP archive of XML parts holding worksheets, shared strings and styles, replacing the binary .xls. It is recognized by '[Content_Types].xml' plus an 'xl/' folder inside the ZIP.
Have a file that might be Excel workbook (xlsx)? Identify it with the File Type Identifier — it reads the magic number locally, nothing is uploaded. Or browse all formats.