Fix content_type validator to support blank/nil

ContentTypeValidator now honors the `:allow_nil` and `:allow_blank` option.

lib/paperclip/validators/attachment_content_type_validator.rb

 module Paperclip
   module Validators
     class AttachmentContentTypeValidator < ActiveModel::EachValidator
+      def initialize(options)
+        options[:allow_nil] = true unless options.has_key?(:allow_nil)
+        super
+      end
+
       def validate_each(record, attribute, value)
         attribute = "#{attribute}_content_type".to_sym
         value = record.send(:read_attribute_for_validation, attribute)
         allowed_types = [options[:content_type]].flatten
 
-        if value.present? && allowed_types.none? { |type| type === value }
+        return if (value.nil? && options[:allow_nil]) || (value.blank? && options[:allow_blank])
+
+        if allowed_types.none? { |type| type === value }
           record.errors.add(attribute, :invalid, options.merge(
             :types => allowed_types.join(', ')
           ))

test/validators/attachment_content_type_validator_test.rb

@@ -24,6 +24,58 @@ class AttachmentContentTypeValidatorTest < Test::Unit::TestCase
     end
   end
 
+  context "with :allow_nil option" do
+    context "as true" do
+      setup do
+        build_validator :content_type => "image/png", :allow_nil => true
+        @dummy.stubs(:avatar_content_type => nil)
+        @validator.validate(@dummy)
+      end
+
+      should "allow avatar_content_type as nil" do
+        assert @dummy.errors[:avatar_content_type].blank?
+      end
+    end
+
+    context "as false" do
+      setup do
+        build_validator :content_type => "image/png", :allow_nil => false
+        @dummy.stubs(:avatar_content_type => nil)
+        @validator.validate(@dummy)
+      end
+
+      should "not allow avatar_content_type as nil" do
+        assert @dummy.errors[:avatar_content_type].present?
+      end
+    end
+  end
+
+  context "with :allow_blank option" do
+    context "as true" do
+      setup do
+        build_validator :content_type => "image/png", :allow_blank => true
+        @dummy.stubs(:avatar_content_type => "")
+        @validator.validate(@dummy)
+      end
+
+      should "allow avatar_content_type as blank" do
+        assert @dummy.errors[:avatar_content_type].blank?
+      end
+    end
+
+    context "as false" do
+      setup do
+        build_validator :content_type => "image/png", :allow_blank => false
+        @dummy.stubs(:avatar_content_type => "")
+        @validator.validate(@dummy)
+      end
+
+      should "not allow avatar_content_type as blank" do
+        assert @dummy.errors[:avatar_content_type].present?
+      end
+    end
+  end
+
   context "with an allowed type" do
     context "as a string" do
       setup do
@@ -48,7 +100,7 @@ class AttachmentContentTypeValidatorTest < Test::Unit::TestCase
         assert @dummy.errors[:avatar_content_type].blank?
       end
     end
-    
+
     context "as a list" do
       setup do
         build_validator :content_type => ["image/png", "image/jpg", "image/jpeg"]