Content-Type
The header declaring a body's format, like application/json.
Also known as: Content-Type header, MIME type, media type, application/json
The Content-Type header says what format the body of a request or response is in, so the receiver knows how to read it.
Content-Type: application/json
Content-Type: text/html; charset=utf-8
Content-Type: multipart/form-data; boundary=----abc123The value is a media type (also called MIME type) such as type/subtype, optionally with parameters like charset.
Common values
| Content-Type | Body |
|---|---|
application/json | JSON (JSON) |
application/x-www-form-urlencoded | classic HTML form fields (a=1&b=2) |
multipart/form-data | forms with file uploads (multipart) |
text/plain, text/html, text/css | text formats |
application/javascript / text/javascript | scripts |
image/png, image/jpeg, image/webp | images |
application/pdf, application/octet-stream | PDFs; unknown binary data |
application/problem+json | structured API errors (error format) |
Why it matters
- Sending JSON without the header makes many servers ignore or reject the body:
fetch("/orders", {
method: "POST",
headers: { "Content-Type": "application/json" },
body: JSON.stringify(order),
});- The browser decides what to do from it: render HTML, run a script, download a file. A wrong type can show raw text, or break styles and scripts.
- Character encoding:
charset=utf-8prevents garbled text (character encoding). - Security: browsers may “sniff” content. Send the right type, and
X-Content-Type-Options: nosniff(security headers). - Servers validate uploads against it, but the client controls the header, so don’t trust it alone (file upload security).
Related
Accept states what the client wants back, and the server picks a format (content negotiation). Set Content-Type on what you send, and read it on what you receive.