Media: string-encoding non-JSON data

New in draft 7

JSON schema has a set of keywords to describe and optionally validate non-JSON data stored inside JSON strings. Since it would be difficult to write validators for many media types, JSON schema validators are not required to validate the contents of JSON strings based on these keywords. However, these keywords are still useful for an application that consumes validated JSON.


The contentMediaType keyword specifies the MIME type of the contents of a string, as described in RFC 2046. There is a list of MIME types officially registered by the IANA, but the set of types supported will be application and operating system dependent. Mozilla Developer Network also maintains a shorter list of MIME types that are important for the web


The contentEncoding keyword specifies the encoding used to store the contents, as specified in RFC 2054, part 6.1 and RFC 4648.

The acceptable values are quoted-printable, base16, base32, and base64. If not specified, the encoding is the same as the containing JSON document.

Without getting into the low-level details of each of these encodings, there are really only two options useful for modern usage:

  • If the content is encoded in the same encoding as the enclosing JSON document (which for practical purposes, is almost always UTF-8), leave contentEncoding unspecified, and include the content in a string as-is. This includes text-based content types, such as text/html or application/xml.
  • If the content is binary data, set contentEncoding to base64 and encode the contents using Base64. This would include many image types, such as image/png or audio types, such as audio/mpeg.


New in draft 2019-09

Documentation Coming soon


The following schema indicates the string contains an HTML document, encoded using the same encoding as the surrounding document:

{ "type": "string", "contentMediaType": "text/html"}
"<!DOCTYPE html><html xmlns=\"\"><head></head></html>"
compliant to schema

The following schema indicates that a string contains a PNG image, encoded using Base64:

{ "type": "string", "contentEncoding": "base64", "contentMediaType": "image/png"}
compliant to schema

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