Make it explicit in JSON Schema that one and only one of the properties are required.

[aslakhellesoy]

The JSON Schema for Envelope doesn't currently emphasise that one and only one of its properties must be defined. I believe the correct way to do this is using oneOf:

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "title": "Demo",
  "oneOf": [
    {
      "properties": {
        "attachment": {
          "$ref": "./Attachment.json"
        }
      },
      "required": ["attachment"]
    },
    {
      "properties": {
        "gherkinDocument": {
          "$ref": "./GherkinDocument.json"
        }
      },
      "required": ["gherkinDocument"]
    },
    {
      "properties": {
        "hook": {
          "$ref": "./Hook.json"
        }
      },
      "required": ["hook"]
    }
  ]
}

(additional properties elided for brevity).