Extract plain file rendering into a separate method

app/helpers/markup_helper.rb

@@ -154,9 +154,7 @@ module MarkupHelper
     elsif asciidoc?(file_name)
       asciidoc_unsafe(text, context)
     elsif plain?(file_name)
-      content_tag :pre, class: 'plain-readme' do
-        text
-      end
+      plain_unsafe(text)
     else
       other_markup_unsafe(file_name, text, context)
     end
@@ -271,6 +269,12 @@ module MarkupHelper
     Gitlab::Asciidoc.render(text, context)
   end
 
+  def plain_unsafe(text)
+    content_tag :pre, class: 'plain-readme' do
+      text
+    end
+  end
+
   def other_markup_unsafe(file_name, text, context = {})
     Gitlab::OtherMarkup.render(file_name, text, context)
   end