Finish the references implementation for the extraction module and update roadmap documentation

docs/integrations/json-output.md

@@ -114,7 +114,7 @@ When references are bound, the manifest section has this shape:
       "origin_type": "file",
       "origin_uri": "file:///absolute/path/roster.txt",
       "digest": "sha256:...",
-      "media_type": "text/plain; charset=utf-8",
+      "media_type": "text/plain",
       "size_bytes": 123,
       "binding_source": "config"
     }
@@ -122,6 +122,10 @@ When references are bound, the manifest section has this shape:
 }
 ```
 
+Reference media types are inferred from file extensions and recorded as
+canonical base media types. Unknown extensions are recorded as
+`application/octet-stream`.
+
 `module_metadata` is omitted when no singleton module provides metadata.
 
 `validation_status` is `approved` when no candidates were rejected and