application/pdf

The document that looks the same everywhere, standardized as ISO 32000 and registered back when Adobe still owned it. Serve it inline and browsers render it; add a Content-Disposition: attachment header and it downloads instead — the type doesn't decide that, the header does. Every real PDF opens with the %PDF- signature, which is why sniffers catch mislabeled ones instantly.

Defined by RFC 8118 · IANA registration. Registry facts from the IANA media-types registry via mime-db.