halo_json_request.json

{
  "_links": {
    "self": { "href": "/foo" },
  },
  "_controls": {
    "attack": {
      "target": "/attacks",
      "method": "POST",
      "headers": {
        "Content-Type": "application/json"
      },
      "schema": { /*_json_schema_doc_here_*/ }
    }
  }
}

halo_xml_request.json

{
  "_links": {
    "self": { "href": "/" },
    "example": { "href": "/example" }
  },
  "_controls": {
    "widgetate": {
      "target": "/widget/{newID}",
      "method": "PUT",
      "headers": {
        "Content-Type": "application/xml"
      },
      "body": "<widget>\\n <name>{{name}}</name>\\n\\n <blobs>\\n {{#blobs}}\\n <blob>\\n {{#first}}\\n <first>true</first>\\n {{/first}}\\n <contents>{{contents}}</contents>\\n </blob>\\n {{/blobs}}\\n </blobs>\\n\\n {{#is_empty}}\\n <note>This is an empty widget</note>\\n {{/is_empty}}\\n</widget>\\n"
    }
  }
}

JSON Hyper-Schema looks like it includes nearly everything in the _controls example above. Notably, it specifies encType and mediaType rather than arbitrary headers.