docs: add x-jsonschema-contentMediaType extension page

registries/_extension/x-jsonschema-contentMediaType.md

@@ -0,0 +1,39 @@
+---
+owner: baywet
+issue:
+description: The JSON Schema contentMediaType annotation, used when targeting OpenAPI versions that do not directly support it.
+schema:
+  type: string
+objects: [ "Schema Object" ]
+layout: default
+---
+
+{% capture summary %}
+JSON Schema defines the `contentMediaType` annotation to describe the media type of a string value's decoded content.
+
+The `x-jsonschema-contentMediaType` extension mirrors this JSON Schema keyword when targeting OpenAPI versions where the keyword is not directly available, serializing it as `x-jsonschema-contentMediaType`.
+
+It can appear as a property in the following objects: `{{page.objects|jsonify}}`.
+
+Used by: (informational)
+
+* [Microsoft.OpenApi](https://github.com/microsoft/OpenAPI.NET) (.NET OpenAPI library)
+{% endcapture %}
+
+{% capture example %}
+```yaml
+openapi: 3.0.4
+info:
+  title: My API
+  version: 1.0.0
+paths: {}
+components:
+  schemas:
+    PngImage:
+      type: string
+      x-jsonschema-contentEncoding: base64
+      x-jsonschema-contentMediaType: image/png
+```
+{% endcapture %}
+
+{% include extension-entry.md summary=summary example=example %}