Help images are not displayed

Help images are not displayed.

For example, https://gitlab.example.com/help/workflow/four_stages.png

Related bugreport in GitLab: https://gitlab.com/gitlab-org/gitlab-ce/issues/765

Some bug also present in omnibus-gitlab 7.9:

# rpm -q gitlab
gitlab-7.9.0_omnibus.2-1.el7.x86_64

# cat /etc/redhat-release
CentOS Linux release 7.0.1406 (Core)

# cat /var/log/gitlab/gitlab-rails/production.log

Started GET "/help/workflow/four_stages.png" for 127.0.0.1 at 2015-03-23 12:10:12 +0200
Processing by HelpController#show as PNG
  Parameters: {"category"=>"workflow", "file"=>"four_stages"}
Completed 500 Internal Server Error in 131ms

ActionView::MissingTemplate (Missing template errors/not_found with {:locale=>[:en], :formats=>[:png], :variants=>[], :handlers=>[:erb, :builder, :raw, :ruby
, :slim, :coffee, :haml]}. Searched in:
  * "/opt/gitlab/embedded/service/gitlab-rails/app/views"
  * "/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/kaminari-0.15.1/app/views"
  * "/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/doorkeeper-2.1.0/app/views"
  * "/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/devise-3.2.4/app/views"
  * "/var/opt/gitlab/gitlab-rails/working"
  * "/"
):
  app/controllers/application_controller.rb:148:in `not_found!'
  app/controllers/help_controller.rb:12:in `show'