The Content-Type entity header is used to indicate the media type of the resource.

Prevent MIME sniffing

Browsers will do MIME sniffing in some cases and will not necessarily follow the value of this header; to prevent this behavior, the header X-Content-Type-Options can be set to nosniff.

MIME sniffing: Content sniffing, also known as media type sniffing or MIME sniffing, is the practice of inspecting the content of a byte stream to attempt to deduce the file format of the data within it.

Syntax

Content-Type: text/html; charset=utf-8
Content-Type: multipart/form-data; boundary=something

Directives

media-type

The MIME type of the resource of the data

MIME type: Multipurpose Internet Mail Extensions (MIME) type is a standard that indicates the nature and format of a document, file, or assortment of bytes.

charset

The character encoding standard

boundary

For multipart entities the boundary directive is required, which consists of 1 to 70 characters from a set of characters known to be very robust through email gateways, and not ending with white space. It is used to encapsulate the boundaries of the multiple parts of the message. Often the header boundary is prepended by two dashes(–) in the body and the final boundary also have a two dashes appended to it.

Content-Type on HTTP POST method

• application/x-www-form-urlencoded: Encoded to key-value tuple. (splitted by ‘&’, connected by ‘=’) Non-alphabet symbol is percent-encoded. So this content type is not suitable to binary data.

• application/form-data: can be used for binary data.
• text/plain

Examples

in HTML forms: enctype

<form action="/" method="post" enctype="multipart/form-data">
  <input type="text" name="description" value="some text">
  <input type="file" name="myFile">
  <button type="submit">Submit</button>
</form>

Parsing multipart/form-data in express.js

Use multer library to parse multipart/form-data.

const multer = require('multuer');

const upload = multer({ storage: multer.memoryStorage() });

router.post('/api/image', upload.single('cell'), (req, res) => {
    res.send(req.file) // payload is stored in `req.file.buffer`
});

• See also: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Type