application/vnd.openxmlformats-officedocument.spreadsheetml.sheet

Excel's Office Open XML workbook (ECMA-376, ISO/IEC 29500): a zip of XML parts, which means the bytes announce a zip and nothing more — a sniffer must look inside for xl/workbook.xml to know it is a spreadsheet rather than a DOCX or an EPUB, so the type deliberately has no magic-byte signature of its own. The everyday mislabel is application/vnd.ms-excel, which properly names the older binary .xls; upload validators that only check the type string wave through either. Send the full unwieldy string exactly — there is no short alias.

.xlsm, the macro-enabled variant, is its own type — application/vnd.ms-excel.sheet.macroEnabled.12.

Defined by ECMA-376 — Office Open XML file formats · IANA registration. Registry facts from the IANA media-types registry via mime-db.