Discuss null values and contentType

Author: handrews

versions/3.0.4.md

@@ -1660,15 +1660,20 @@ Field Name | Type | Description
 
 This object MAY be extended with [Specification Extensions](#specificationExtensions).
 
-The default values for `contentType` are as follows:
+The default values for `contentType` are as follows, where an _n/a_ in the `format` column means that the presence or value of `format` is irrelevant:
 
-Property Type | Property Format | Default `contentType`
+Property `type` | Property `format` | Default `contentType`
 ------------- | --------------- | ---------------------
 `string` | `binary` | `application/octet-stream`
-`string`, `number`, `integer`, or `boolean` | _n/a_ | `text/plain`
+`string` | _none, or any except `binary`_ | `text/plain`
+`number`, `integer`, or `boolean` | _n/a_ | `text/plain`
 `object` | _n/a_ | `application/json`
 `array` | _n/a_ | according to the `type` and `format` of the `items` schema
 
+Determining how to handle `null` values if `nullable: true` is present depends on how `null` values are being serialized.
+If `null` values are entirely omitted, then the `contentType` is irrelevant.
+See [Appendix B](#dataTypeConversion) for a discussion of data type conversion options.
+
 ##### Fixed Fields for RFC6570-style Serialization
 
 Field Name | Type | Description