MIME Type Lookup

Both work, but the WHATWG HTML spec officially recommends text/javascript when serving JavaScript over HTTP. application/javascript is widely supported and you'll see it in older configurations.

Only image/jpeg is the registered IANA type. image/jpg is a common misspelling — most browsers tolerate it but tools and HTTP caches may not.

Use it for arbitrary binary data when no more specific type fits, or to force the browser to download the file instead of displaying it. Pair with a Content-Disposition: attachment header.

It usually does not — the modern default is to trust the declared type, which is why a mislabelled file fails rather than being rescued. Where a browser does look at the bytes, it is for a narrow set of cases, and X-Content-Type-Options: nosniff turns that off entirely. A stylesheet or a module script served with the wrong type is refused outright, which is the most common way this shows up.

It tells the client how to decode the bytes of a text type, and it outranks anything the document says about itself — a page declaring UTF-8 in a meta tag still renders as mojibake if the header says otherwise. Binary types have no charset, and adding one is meaningless; text/plain without one leaves the decoding to a guess.