MIME type lookup

What a MIME type is and why it matters

A MIME type, also called a media type or content type, is a short label such as image/png or application/pdf that says what kind of data a file holds. Browsers, servers, email programs and APIs use it to decide what to do with the data: show an image, download a document, run a script or reject a file. Getting it right avoids strange problems such as a stylesheet that is ignored, a font that will not load or a PDF that downloads instead of opening.

Where you need them

• Server configuration. A web server sends a Content-Type header with every file. If the type is missing or wrong, the browser may refuse to use it, for example a script served as plain text.
• File uploads. Forms and APIs often accept only certain types, such as image/jpeg and image/png. The accept attribute of a file input takes MIME types as well as extensions.
• APIs. Requests and responses declare their format with headers like Content-Type and Accept.
• Email attachments and downloads. Each part of a message carries its type.

Extension and type are not the same

A file's extension is just part of its name, and anyone can change it. The MIME type is what the server declares for the file, and what a browser actually trusts. That is why a file renamed from .exe to .jpg does not become an image. For security, servers can add the header X-Content-Type-Options: nosniff, which tells the browser to obey the declared type and not guess.

A few useful notes

For text types it is good practice to add a character set, as in text/html; charset=utf-8. JavaScript is now officially text/javascript, although the older application/javascript is still common. When a server does not know the type of a file, it falls back to application/octet-stream, which browsers treat as a download. Some files have several extensions for the same type, such as .jpg and .jpeg.

What this list covers

It lists the types web developers and site owners meet most often, around ninety. It is not the full registry kept by IANA, which has thousands of entries, and some types, such as those for the Office formats, are long by nature. Check the official registry for a rare format.