application/vnd.openxmlformats-officedocument.wordprocessingml.document

Word's format since 2007 and the reason the type string is a paragraph long: Office Open XML, standardized as ECMA-376 and ISO/IEC 29500. A .docx is a plain zip of XML parts, so a byte sniffer sees only the PK header — telling it apart from XLSX, PPTX or EPUB means opening the archive and reading [Content_Types].xml or spotting word/document.xml, which is why this type carries no signature of its own. Servers routinely mislabel it application/msword (the binary .doc type) or give up and send application/octet-stream; both break content negotiation.

.docm, the macro-enabled variant, is the same zip under a different type — application/vnd.ms-word.document.macroEnabled.12.

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