{
    "title": "Structure of Attachment",
    "type": "object",
    "properties":
    {
      "link":           { "type": "string" },
      "k":              { "type": "string" },  
      "hash":           { "type": "string" },  
      "size":           { "type": "integer" }
     },
     "required": ["link", "k", "hash", "size"]
  }