XLSX
Spreadsheets as a zip of XML.
Where it came from
XLSX arrived with Office 2007 as part of Office Open XML — Microsoft moved binary formats (.xls, .doc, .ppt) to 'a zip with XML inside', partially opening the specs under competitive and government pressure. Since 2008 it's standardized as ISO/IEC ECMA-376.
Today it's understood not only by Excel but by LibreOffice, Google Sheets, Numbers and half the BI tooling.
How it works
Physically an .xlsx is a ZIP archive: workbook.xml describes sheets, sheet1.xml stores cells (referencing shared strings in sharedStrings.xml), styles.xml holds formatting. That's why a corrupted xlsx can sometimes be 'repaired' by unzipping it.
Cells are typed: numbers, strings (via shared-string indices), dates (numbers with a format), formulas stored alongside their last computed value.
UWC reads XLSX via exceljs (MIT) on the server and outputs CSV or JSON — files are processed in memory and never stored.
Convert XLSX
Excel sheets to CSV for scripts or JSON for code — without installing Office.