Vendor extension on array parameter items node

This might not be currently allows on the items node per https://swagger.io/docs/specification/2-0/swagger-extensions/

I came across this:

        "parameters": [
          {
            "name": "parameterName",
            "in": "body",
            "description": "Parameter description",
            "required": true,
            "schema": {
              "type": "array",
              "items": {
                "type": "string",
                "x-my-vendor-extension": {

 The vendor extension here was used to provide a GUI picker for the items inside the array, where the picker returns a single item, rather than the whole array.

Could it be the above structure is currently an out-of-spec placement for the vendor extension?