application/zip

The archive format so successful it became infrastructure: DOCX, XLSX, PPTX, ODT, EPUB, APK and JAR are all zip files wearing different labels. Send application/zip only for an actual .zip download — anything more specific deserves its own type, because a sniffer that sees the PK header knows only that it found a zip, not which of a dozen formats it is. The unregistered application/x-zip-compressed still shows up from Windows uploads, and note the payload is already deflated, so compressing it again on the wire buys nothing.

Countless formats hide a zip behind their own extension; only .zip itself should be served with this type.

Defined by PKWARE APPNOTE.TXT — .ZIP File Format Specification · IANA registration. Registry facts from the IANA media-types registry via mime-db.