application/vnd.oasis.opendocument.text

The OpenDocument text format — LibreOffice's native document, standardized by OASIS and published as ISO/IEC 26300. Like the OOXML formats it is a zip of XML, so the bytes alone say PK and nothing else; ODF's packaging spec answers that neatly by requiring a file named mimetype, stored first and uncompressed, so the full type string sits in plain sight near the start of the archive. Sniffers that read that far can identify ODT precisely, which is more than the zip signature will ever tell them. Mislabels usually round it off to application/zip or application/octet-stream.

.ott templates are a separate type — application/vnd.oasis.opendocument.text-template.

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