create a private url generator when the private visibility option is set

lib/paperclip/storage/dropbox/generator_factory.rb

@@ -5,7 +5,7 @@ module Storage
     module Dropbox
       module GeneratorFactory
         def self.build_url_generator(storage, options)
-          if options[:dropbox_credentials][:access_type] == "app_folder"
+          if options[:dropbox_credentials][:access_type] == "app_folder" || options[:dropbox_visibility] == "private"
             PrivateUrlGenerator.new(storage, options)
           elsif options[:dropbox_credentials][:access_type] == "dropbox"
             PublicUrlGenerator.new(storage, options)

lib/paperclip/storage/dropbox/public_url_generator.rb

@@ -6,7 +6,7 @@ class PublicUrlGenerator < UrlGenerator
         def file_url(style)
           url = URI.parse("https://dl.dropboxusercontent.com/u/#{user_id}/")
           path = @attachment.path(style)
-          path = path.match(/^Public\//).post_match
+          path = path.match(/^Public\//).try(:post_match)
           url.merge!(path)
           url.to_s
         end

spec/lib/paperclip/storage/dropbox/generator_factor_spec.rb

@@ -0,0 +1,22 @@
+require "spec_helper"
+require "paperclip/storage/dropbox/generator_factory"
+
+describe Paperclip::Storage::Dropbox::GeneratorFactory do
+  it "should build a private url generator" do
+    described_class.build_url_generator(double(:storage), {
+      dropbox_credentials: {access_type: "app_folder"}}).should be_a(Paperclip::Storage::Dropbox::PrivateUrlGenerator)
+  end
+
+  it "should build a private url generator" do
+     described_class.build_url_generator(double(:storage), {
+       dropbox_credentials: { access_type: "dropbox" },
+       dropbox_visibility: 'private'}).should be_a(Paperclip::Storage::Dropbox::PrivateUrlGenerator)
+  end
+
+  it "should build a public url generator" do
+     described_class.build_url_generator(double(:storage), {
+       dropbox_credentials: { access_type: "dropbox" },
+       dropbox_visibility: 'public'}).should be_a(Paperclip::Storage::Dropbox::PublicUrlGenerator)
+  end
+end
+