application/vnd.oasis.opendocument.spreadsheet

The OpenDocument spreadsheet, OASIS-standardized and mirrored as ISO/IEC 26300 — the one spreadsheet format whose formula language is itself a published standard, OpenFormula, rather than whatever the vendor's engine happens to do. The file is a zip package, so there is no signature to list: the PK bytes identify the container, and ODF's requirement that an uncompressed file named mimetype come first in the archive is what lets tools read the exact type straight out of the early bytes. Serve the full type string; falling back to application/zip loses the ability to open it in place.

.ots templates are a separate type — application/vnd.oasis.opendocument.spreadsheet-template.

Defined by OASIS OpenDocument Format v1.3 · IANA registration. Registry facts from the IANA media-types registry via mime-db.