application/octet-stream

The type that means no type: arbitrary bytes with no promise about what they are. Browsers respond by refusing to render and offering a download instead, which is why a misconfigured server makes PDFs and images save to disk. It is also the sniffing loophole — user agents may guess the real type from the bytes, so an untyped upload can come back interpreted as HTML unless you send X-Content-Type-Options: nosniff alongside it. Reach for it as a last resort when the type is genuinely unknown; if you know what the bytes are, say so.

The extension list is a grab-bag by design — .bin, .exe, .dmg, .iso and friends all land here because nothing more specific claims them.

Defined by RFC 2046 · IANA registration. Registry facts from the IANA media-types registry via mime-db.