Use correct upload endpoint for group wikis

This URL is passed on to the JS frontend which uses it to generate
upload requests.

app/helpers/wiki_helper.rb

@@ -69,7 +69,12 @@ module WikiHelper
   end
 
   def wiki_attachment_upload_url
-    expose_url(api_v4_projects_wikis_attachments_path(id: @wiki.container.id))
+    case @wiki.container
+    when Project
+      expose_url(api_v4_projects_wikis_attachments_path(id: @wiki.container.id))
+    else
+      raise TypeError, "Unsupported wiki container #{@wiki.container.class}"
+    end
   end
 
   def wiki_sort_controls(wiki, sort, direction)
@@ -147,3 +152,5 @@ module WikiHelper
       !container.has_confluence?
   end
 end
+
+WikiHelper.prepend_if_ee('EE::WikiHelper')

ee/app/helpers/ee/wiki_helper.rb

+# frozen_string_literal: true
+
+module EE
+  module WikiHelper
+    extend ::Gitlab::Utils::Override
+
+    override :wiki_attachment_upload_url
+    def wiki_attachment_upload_url
+      case @wiki.container
+      when Group
+        expose_url(api_v4_groups_wikis_attachments_path(id: @wiki.container.id))
+      else
+        super
+      end
+    end
+  end
+end

ee/spec/helpers/ee/wiki_helper_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe WikiHelper do
+  describe '#wiki_attachment_upload_url' do
+    it 'returns the upload endpoint for group wikis' do
+      @wiki = build_stubbed(:group_wiki)
+
+      expect(helper.wiki_attachment_upload_url).to end_with("/api/v4/groups/#{@wiki.group.id}/wikis/attachments")
+    end
+  end
+end

spec/helpers/wiki_helper_spec.rb

@@ -53,6 +53,22 @@ RSpec.describe WikiHelper do
     end
   end
 
+  describe '#wiki_attachment_upload_url' do
+    it 'returns the upload endpoint for project wikis' do
+      @wiki = build_stubbed(:project_wiki)
+
+      expect(helper.wiki_attachment_upload_url).to end_with("/api/v4/projects/#{@wiki.project.id}/wikis/attachments")
+    end
+
+    it 'raises an exception for unsupported wiki containers' do
+      @wiki = Wiki.new(User.new)
+
+      expect do
+        helper.wiki_attachment_upload_url
+      end.to raise_error(TypeError)
+    end
+  end
+
   describe '#wiki_sort_controls' do
     let(:wiki) { create(:project_wiki) }
     let(:wiki_link) { helper.wiki_sort_controls(wiki, sort, direction) }