Assigning an empty string does nothing

lib/paperclip.rb

@@ -181,6 +181,7 @@ end
 # This stuff needs to be run after Paperclip is defined.
 require 'paperclip/io_adapters/registry'
 require 'paperclip/io_adapters/abstract_adapter'
+require 'paperclip/io_adapters/empty_string_adapter'
 require 'paperclip/io_adapters/identity_adapter'
 require 'paperclip/io_adapters/file_adapter'
 require 'paperclip/io_adapters/stringio_adapter'

lib/paperclip/attachment.rb

@@ -93,6 +93,7 @@ module Paperclip
       ensure_required_accessors!
       file = Paperclip.io_adapters.for(uploaded_file)
 
+      return nil if not file.assignment?
       self.clear(*only_process)
       return nil if file.nil?
 

lib/paperclip/io_adapters/abstract_adapter.rb

@@ -23,6 +23,14 @@ module Paperclip
       @original_filename = new_filename.gsub(OS_RESTRICTED_CHARACTERS, "_")
     end
 
+    def nil?
+      false
+    end
+
+    def assignment?
+      true
+    end
+
     private
 
     def destination

lib/paperclip/io_adapters/empty_string_adapter.rb

+module Paperclip
+  class EmptyStringAdapter < AbstractAdapter
+    def initialize(target)
+    end
+
+    def nil?
+      false
+    end
+
+    def assignment?
+      false
+    end
+  end
+end
+
+Paperclip.io_adapters.register Paperclip::EmptyStringAdapter do |target|
+  target.is_a?(String) && target.empty?
+end

test/io_adapters/abstract_adapter_test.rb

@@ -47,4 +47,12 @@ class AbstractAdapterTest < Test::Unit::TestCase
 
     assert_equal "awesome_file_name.png", @adapter.original_filename
   end
+
+  should 'be an assignment' do
+    assert TestAdapter.new.assignment?
+  end
+
+  should 'not be nil' do
+    assert !TestAdapter.new.nil?
+  end
 end

test/io_adapters/empty_string_adapter_test.rb

+require './test/helper'
+
+class EmptyStringAdapterTest < Test::Unit::TestCase
+  context 'a new instance' do
+    setup do
+      @subject = Paperclip.io_adapters.for('')
+    end
+
+    should "return false for a call to nil?" do
+      assert !@subject.nil?
+    end
+
+    should 'return false for a call to assignment?' do
+      assert !@subject.assignment?
+    end
+  end
+end