Fix explicit names on multiple file fields

If a file field tag is passed the multiple option, it is turned into an
array field (appending "[]"), but if the file field is passed an
explicit name as an option, leave the name alone (do not append "[]").

Fixes #9830

actionpack/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Fix explicit names on multiple file fields. If a file field tag is passed
+    the multiple option, it is turned into an array field (appending `[]`),
+    but if the file field is passed an explicit name as an option, leave the
+    name alone (do not append `[]`).  Fixes #9830
+
+    *Ryan McGeary*
+
 *   Add block support for the `mail_to` helper, similar to the `link_to` helper.
 
     *Sam Pohlenz*

actionpack/lib/action_view/helpers/tags/base.rb

@@ -73,27 +73,26 @@ def add_default_name_and_id_for_value(tag_value, options)
 
         def add_default_name_and_id(options)
           if options.has_key?("index")
-            options["name"] ||= options.fetch("name"){ tag_name_with_index(options["index"]) }
+            options["name"] ||= options.fetch("name"){ tag_name_with_index(options["index"], options["multiple"]) }
             options["id"] = options.fetch("id"){ tag_id_with_index(options["index"]) }
             options.delete("index")
           elsif defined?(@auto_index)
-            options["name"] ||= options.fetch("name"){ tag_name_with_index(@auto_index) }
+            options["name"] ||= options.fetch("name"){ tag_name_with_index(@auto_index, options["multiple"]) }
             options["id"] = options.fetch("id"){ tag_id_with_index(@auto_index) }
           else
-            options["name"] ||= options.fetch("name"){ tag_name }
+            options["name"] ||= options.fetch("name"){ tag_name(options["multiple"]) }
             options["id"] = options.fetch("id"){ tag_id }
           end
 
-          options["name"] += "[]" if options["multiple"] && !options["name"].ends_with?("[]")
           options["id"] = [options.delete('namespace'), options["id"]].compact.join("_").presence
         end
 
-        def tag_name
-          "#{@object_name}[#{sanitized_method_name}]"
+        def tag_name(multiple = false)
+          "#{@object_name}[#{sanitized_method_name}]#{"[]" if multiple}"
         end
 
-        def tag_name_with_index(index)
-          "#{@object_name}[#{index}][#{sanitized_method_name}]"
+        def tag_name_with_index(index, multiple = false)
+          "#{@object_name}[#{index}][#{sanitized_method_name}]#{"[]" if multiple}"
         end
 
         def tag_id

actionpack/test/template/form_helper_test.rb

@@ -361,6 +361,16 @@ def test_file_field_has_no_size
     assert_dom_equal expected, file_field("user", "avatar")
   end
 
+  def test_file_field_with_multiple_behavior
+    expected = '<input id="import_file" multiple="multiple" name="import[file][]" type="file" />'
+    assert_dom_equal expected, file_field("import", "file", :multiple => true)
+  end
+
+  def test_file_field_with_multiple_behavior_and_explicit_name
+    expected = '<input id="import_file" multiple="multiple" name="custom" type="file" />'
+    assert_dom_equal expected, file_field("import", "file", :multiple => true, :name => "custom")
+  end
+
   def test_hidden_field
     assert_dom_equal(
       '<input id="post_title" name="post[title]" type="hidden" value="Hello World" />',