MIME Type

The standard label for a file's format, like image/png.

Backend Engineer · Junior Frontend Engineer · Junior

Also known as: media type, content type

A MIME type is a standard label that describes a file’s format, written as a type and a subtype separated by a slash. Examples include text/html, image/png and application/json. Browsers and servers use it to decide how to handle the bytes they receive.

Servers send the MIME type in the Content-Type header of a response:

HTTP/1.1 200 OK
Content-Type: image/png

When a browser receives text/html, it renders the content as a page. When it receives image/png, it displays the bytes as an image. When it receives application/octet-stream, it usually offers a download, because the format is unknown.

The classic mistake is trusting the file extension or the MIME type a client sends when accepting an upload. A file named photo.png can contain anything, and a client can claim any type it likes. Check the file’s actual contents before you accept it, as covered in file uploads, and store the type you verified rather than the one you were given.