Explain lack of HTTP header for base64 encoding

Also, remove the example that goes against the advice in the
updated binary-handling section.

Author: handrews

versions/3.1.1.md

@@ -174,7 +174,7 @@ Keyword | Raw | Encoded | Comments
 `contentMediaType` | `image/png` | `image/png` | can sometimes be omitted if redundant (see below)
 `contentEncoding`  | _omit_ | `base64` or `base64url` | other encodings are [allowed](https://datatracker.ietf.org/doc/html/draft-bhutton-json-schema-validation-00#section-8.3)
 
-Note that the encoding indicated by `contentEncoding`, which inflates the size of data in order to represent it as 7-bit ASCII text, is unrelated to HTTP's `Content-Encoding` header, which indicates whether and how a message body has been compressed and is applied after all content serialization described in this section has occurred.
+Note that the encoding indicated by `contentEncoding`, which inflates the size of data in order to represent it as 7-bit ASCII text, is unrelated to HTTP's `Content-Encoding` header, which indicates whether and how a message body has been compressed and is applied after all content serialization described in this section has occurred.  Since HTTP allows unencoded binary message bodies, there is no standardized HTTP header for indicating base64 or similar encoding of an entire message body.
 
 Using a `contentEncoding` of `base64url` ensures that URL encoding (as required in the query string and in message bodies of type `application/x-www-form-urlencoded`) does not need to further encode any part of the already-encoded binary data.
 
@@ -1499,19 +1499,6 @@ content:
   application/octet-stream: {}
 ```
 
-Binary content transferred with base64 encoding:
-
-```yaml
-content:
-  image/png:
-    schema:
-      type: string
-      contentMediaType: image/png
-      contentEncoding: base64
-```
-
-Note that the `Content-Type` remains `image/png`, describing the semantics of the payload.  The JSON Schema `type` and `contentEncoding` fields explain that the payload is transferred as text.  The JSON Schema `contentMediaType` is technically redundant, but can be used by JSON Schema tools that may not be aware of the OpenAPI context.
-
 These examples apply to either input payloads of file uploads or response payloads.
 
 A `requestBody` for submitting a file in a `POST` operation may look like the following example: