Look up the MIME type for a file extension, or extensions for a MIME type, online.
Find the MIME (content) type for a file extension, or go the other way and list the extensions for a MIME type. Useful for setting Content-Type headers correctly.
They tell browsers and servers how to handle a file. Serving JSON as text/plain or a font with the wrong type can break things.
Type an extension like "png" to get image/png, or type "image/png" to get its extensions.