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v3.2: Provide guidance for multiple contentType values #4744

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11 changes: 10 additions & 1 deletion src/oas.md
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Expand Up @@ -1768,10 +1768,19 @@ See [Encoding Usage and Restrictions](#encoding-usage-and-restrictions) for guid

Note that there are significant restrictions on what headers can be used with `multipart` media types in general ([RFC2046](https://www.rfc-editor.org/rfc/rfc2046.html#section-5.1)) and `multi-part/form-data` in particular ([RFC7578](https://www.rfc-editor.org/rfc/rfc7578.html#section-4.8)).

###### Handling Multiple `contentType` Values

When multiple values are provided for `contentType`, parsing remains straightforward as the part's actual `Content-Type` is included in the document.

For encoding and serialization, implementations MUST provide a mechanism for applications to indicate which media type is intended.
Implementations MAY choose to offer media type sniffing ([[!SNIFF]]) as an alternative, but this MUST NOT be the default behavior due to the security risks inherent in the process.

###### `Content-Transfer-Encoding` and `contentEncoding`

Note also that `Content-Transfer-Encoding` is deprecated for `multipart/form-data` ([RFC7578](https://www.rfc-editor.org/rfc/rfc7578.html#section-4.7)) where binary data is supported, as it is in HTTP.

Using `contentEncoding` for a multipart field is equivalent to specifying an [Encoding Object](#encoding-object) with a `headers` field containing `Content-Transfer-Encoding` with a schema that requires the value used in `contentEncoding`.
+If `contentEncoding` is used for a multipart field that has an Encoding Object with a `headers` field containing `Content-Transfer-Encoding` with a schema that disallows the value from `contentEncoding`, the result is undefined for serialization and parsing.
If `contentEncoding` is used for a multipart field that has an Encoding Object with a `headers` field containing `Content-Transfer-Encoding` with a schema that disallows the value from `contentEncoding`, the result is undefined for serialization and parsing.

Note that as stated in [Working with Binary Data](#working-with-binary-data), if the Encoding Object's `contentType`, whether set explicitly or implicitly through its default value rules, disagrees with the `contentMediaType` in a Schema Object, the `contentMediaType` SHALL be ignored.
Because of this, and because the Encoding Object's `contentType` defaulting rules do not take the Schema Object's`contentMediaType` into account, the use of `contentMediaType` with an Encoding Object is NOT RECOMMENDED.
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