File Extension by MIME Type Lookup

The MIME type is what tells a browser what to do with the content it received. It travels in Content-Type and has no required relationship to the filename extension — the server decides. When the two disagree, the familiar problems appear: the CSS that is not applied, the script the browser refuses to run, the PDF that opens as text.

Enter the type and the page returns the conventional matching extension, covering what shows up in day-to-day web work: text and data formats, images, fonts, the most common audio and video, and wasm. It is the lookup that solves the practical case of saving a file under the right name when all you have is the response header.

Two things to keep in mind. The relationship is not one to one: the same type accepts more than one extension, such as jpg and jpeg, and the same extension has travelled under different names over the years — JavaScript was text/javascript, then application/javascript, and the current specification went back to text/javascript. And the header can carry parameters after a semicolon, such as charset, which are not part of the type itself.