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The type of last resort and of first resort: logs, READMEs, and anything a server can't otherwise identify. Its charset story is the oldest trap on the web — the default is still US-ASCII by rule (RFC 2046, kept in place by RFC 6657), so send charset=utf-8 explicitly or anything beyond English is technically mislabeled. Browsers used to sniff text/plain responses for hidden HTML, which is exactly the hole X-Content-Type-Options: nosniff was invented to close.
Defined by RFC 2046 · IANA registration. Registry facts from the IANA media-types registry via mime-db.