file_field.patch

diff --git a/actionpack/lib/action_view/helpers/form_helper.rb b/actionpack/lib/action_view/helpers/form_helper.rb
index 79a9d99..97dd1f8 100644
--- a/actionpack/lib/action_view/helpers/form_helper.rb
+++ b/actionpack/lib/action_view/helpers/form_helper.rb
@@ -317,8 +317,10 @@ module ActionView
         options[:html] ||= {}
         options[:html][:remote] = options.delete(:remote)
 
-        output  = form_tag(options.delete(:url) || {}, options.delete(:html) || {})
-        output << fields_for(object_name, object, options, &proc)
+        fields_for = fields_for(object_name, object, options, &proc)
+        default_opts = @multipart ? { :multipart => true } : {}
+        output = form_tag(options.delete(:url) || {}, (options.delete(:html) || {}).merge(default_opts))
+        output << fields_for
         output.safe_concat('</form>')
       end
 
@@ -684,6 +686,7 @@ module ActionView
       #   # => <input type="file" id="attachment_file" name="attachment[file]" class="file_input" />
       #
       def file_field(object_name, method, options = {})
+        @multipart = true
         InstanceTag.new(object_name, method, self, options.delete(:object)).to_input_field_tag("file", options.update({:size => nil}))
       end
 
@@ -871,6 +874,7 @@ module ActionView
             raise ArgumentError, "object[] naming but object param and @object var don't exist or don't respond to to_param: #{object.inspect}"
           end
         end
+        @multipart = nil
       end
 
       def to_label_tag(text = nil, options = {}, &block)