MEDIA-TYPES is a library for querying and comparing media types.

Probably the most useful function is extension-media-type:

(extension-media-type "txt") => "text/plain"

An unknown extension, or no extension, has a media type of application/octet-stream:

(extension-media-type "my-fake-extension") => "application/octet-stream"
(extension-media-type nil) => "application/octet-stream"

(This is nonstandard, but reflects general practice.)

You can also use media-type-extension to query the extension (or extensions) for a media type:

(media-type-extension "application/javascript") => "js"

If a media type has multiple extensions, they are returned as multiple values:

(media-type-extension "image/jpeg") => "jpeg", "jpg", "jpe"

Extensions are not case sensitive:

(extension-media-type "TXT") => "text/plain"

You may pass the extension with or without a leading dot:

(extension-media-type ".txt") => "text/plain"

You can get the extension of a file name with file-name-extension:

(file-name-extension "file.txt") => "txt"

Or just get the media type of the file directly:

(file-name-media-type "file.txt") => "text/plain"
(file-name-media-type #p"file.txt") => "text/plain"

A file name without an extension has a media type of application/octet-stream:

(file-name-media-type "file") => "application/octet-stream"

However, if you want to test whether a file is of a specific media type, you should use file-name-media-typep.

(file-name-media-typep "file.xml" "application/xml") => t
(file-name-media-typep "file.svg" "application/xml") => t

file-name-media-typep uses media-subtypep, which has a sophisticated understanding of media types:

(media-subtypep "text/plain" "*/*") => t
(media-subtypep "text/plain" "text/*") => t

;; All media types are effectively subtypes of application/octet-stream.
(media-subtypep "text/plain" "application/octet-stream") => t

;; Structured syntax suffixes.
(media-subtypep "image/svg+xml" "application/xml") => t
(media-subtypep "application/vnd.my-format+json" "application/json") => t

;; Media type parameters.
(media-subtypep "application/atom+xml;type=entry" "application/xml") => t
(media-subtypep "application/atom+xml" "application/atom+xml;type=entry") => nil
(media-subtypep "application/atom+xml;charset=UTF8;type=entry"
                "application/atom+xml;type=entry")
=> t

;; The "q" pseudo-parameter is used in HTTP content negotiation.
(media-subtypep "application/json" "*/*;q=0.1") => t

MEDIA-TYPES uses the mime.types from the Apache sources.