class UrlUpload
    EXTENSIONS = {
      "image/jpeg" => ["jpg", "jpeg", "jpe"],
      "image/gif" => ["gif"],
      "image/png" => ["png"]
    }
    attr_reader :original_filename, :attachment_data
    def initialize(url)
      @attachment_data = open(url)
      @original_filename = determine_filename
    end

    # Pass things like size, content_type, path on to the downloaded file
    def method_missing(symbol, *args)
      if self.attachment_data.respond_to? symbol
        self.attachment_data.send symbol, *args
      else
        super
      end
    end
    
    private
      def determine_filename
        # Grab the path - even though it could be a script and not an actual file
        path = self.attachment_data.base_uri.path
        # Get the filename from the path, make it lowercase to handle those
        # crazy Win32 servers with all-caps extensions
        filename = File.basename(path).downcase
        # If the file extension doesn't match the content type, add it to the end, changing any existing .'s to _
        filename = [filename.gsub(/\./, "_"), EXTENSIONS[self.content_type].first].join(".") unless EXTENSIONS[self.content_type].any? {|ext| filename.ends_with?("." + ext) }
        # Return the result
        filename
      end
  end

@model.uploaded_data = UrlUpload.new(url)

def url=(value)
  self.uploaded_data = UrlUpload.new(value)
end