The Tool Desk
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How an Internet media type is structured
RFC 6838 describes a media type as a label for content carried by Internet protocols. A registered media type has a top-level type and a subtype; together, those names identify the media type. Optional parameters can add information when the particular type defines them.
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- Top-level type: groups content into a broad category, such as
imageortext. - Subtype: identifies a format or meaning within that category, such as
pnginimage/png. - Parameter: supplies type-specific information after a semicolon, as in
charset=utf-8intext/plain; charset=utf-8.
Parameters are not universal. A media type’s registration specifies any parameters it defines, their names and values, and what they mean. Parameter names are case-insensitive, and their order does not convey meaning. Do not assume, for example, that every media type accepts charset.
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Protocols such as HTTP and MIME use media types to label content so receiving software can interpret or process it under the applicable format and protocol rules. The label identifies the content type; it does not by itself guarantee that a particular browser, mail client, or operating system can handle it.
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Media type and the Content-Type field
Content-Type is a protocol header field commonly used to carry a media type value. The field name and the value are related but not interchangeable: Content-Type names the field, while a value such as application/json is a media type.
Media type and file extension
A media type is a protocol-level type-and-subtype label; a file extension is a filename convention. An extension may be listed as supplementary information in a registration, but it is not the media type itself.
What the top-level categories mean
IANA’s Top-Level Media Types registry lists these eleven categories: application, audio, example, font, haptics, image, message, model, multipart, text, and video. The list is a current registry snapshot, not a permanent limit; check IANA for the live categories and their defining specifications.
The example category is reserved for illustrative examples, not ordinary content-type registrations. IANA links each top-level category to a defining RFC and a subtype registry.
How to check whether a media type is registered
- Open the IANA Media Types registry, the authoritative directory of registered media types and subtypes.
- Find the relevant top-level category, then look for the subtype and its registration record or linked specification.
- Check the spelling, status, and documented parameters in the live record before relying on a value. The registry changes; IANA’s page showed a last-updated date of September 24, 2026, when checked for this article.
IANA’s registry note says: “Media Types (formerly known as MIME types) and Media Subtypes will be assigned and listed by the IANA.”
How registration works—and what it does not promise
RFC 6838 sets out media-type registration procedures, while RFC 9694 updates its guidance for defining new top-level types. IANA’s registry points to the relevant procedures and records; its media type application asks a proposer for the type and subtype, any required parameters, a published specification, intended application usage, fragment-identifier considerations, restrictions, and other information.
The process depends on the registration tree, and standards-tree registrations are subject to standards-related review. Registration is not a guarantee of universal software support: RFC 6838 does not require every application to implement every registered type. If a type is intended for limited use, that restriction should be documented.
Names and syntax at a glance
Registered type and subtype names follow restricted syntax and are case-insensitive. In the standards tree, a dot in a subtype can indicate a registration facet, and a final plus can introduce a structured syntax suffix. These conventions are part of registered-name syntax, not substitutes for checking the registration record and its specification.
For the formal rules, see RFC 6838, Media Type Specifications and Registration Procedures, and RFC 9694, Guidelines for the Definition of New Top-Level Media Types.
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