I have become quite fond of the JSON mapping approach in No-Magic JSON Parsing with Swift Part 2 and extended it with the following convenience method.

extension JSONObject {
    /// Returns a JSON object initialized by reading into it the data from the file specified by a given path.
    /// - Parameter path: The absolute path of the file from which to read data.
    /// - Returns: A JSON object initialized by reading into it the data from the file specified by path.
    convenience init?(contentsOfFile path: String) {
        guard let data = NSData(contentsOfFile: path),
                  json = (try? NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments)) as? [String : AnyObject] else { return nil }
        self.init(dictionary: json)
    }
}

You can also find on GitHub as a Gist