Merge pull request #9745 from madmax/allow_pass_multipart_to_form_for

Fix regression: allow pass multipart option to form_for

actionpack/lib/action_view/helpers/form_helper.rb

@@ -433,7 +433,7 @@ def form_for(record, options = {}, &block)
 
         builder = instantiate_builder(object_name, object, options)
         output  = capture(builder, &block)
-        html_options[:multipart] = builder.multipart?
+        html_options[:multipart] ||= builder.multipart?
 
         form_tag(options[:url] || {}, html_options) { output }
       end

actionpack/test/template/form_helper_test.rb

@@ -2791,8 +2791,8 @@ def test_form_for_with_labelled_builder_with_nested_fields_for_with_custom_build
   end
 
   def test_form_for_with_html_options_adds_options_to_form_tag
-    form_for(@post, html: { id: 'some_form', class: 'some_class' }) do |f| end
-    expected = whole_form("/posts/123", "some_form", "some_class", method: "patch")
+    form_for(@post, html: { id: 'some_form', class: 'some_class', multipart: true }) do |f| end
+    expected = whole_form("/posts/123", "some_form", "some_class", method: "patch", multipart: "multipart/form-data")
 
     assert_dom_equal expected, output_buffer
   end