added a reverse index of user uploads + rake task

spec/models/upload_spec.rb

@@ -5,6 +5,8 @@ describe Upload do
   it { should belong_to :user }
   it { should belong_to :topic }
 
+  it { should have_and_belong_to_many :post }
+
   it { should validate_presence_of :original_filename }
   it { should validate_presence_of :filesize }
 
@@ -38,7 +40,7 @@ describe Upload do
     end
 
     context "s3" do
-      before(:each) do 
+      before(:each) do
         SiteSetting.stubs(:enable_s3_uploads?).returns(true)
         S3.stubs(:store_file).returns(url)
       end