Factor markup? || gitlab_markdown? into new method

app/helpers/tree_helper.rb

@@ -90,7 +90,7 @@ module TreeHelper
   end
 
   def editing_preview_title(filename)
-    if gitlab_markdown?(filename) || markup?(filename)
+    if Gitlab::MarkdownHelper.previewable?(filename)
       'Preview'
     else
       'Diff'

app/models/tree.rb

@@ -15,7 +15,7 @@ class Tree
       # by markup renderer.
       if available_readmes.length > 1
         supported_readmes = available_readmes.select do |readme|
-          gitlab_markdown?(readme.name) || markup?(readme.name)
+          previewable?(readme.name)
         end
 
         # Take the first supported readme, or the first available readme, if we

lib/gitlab/markdown_helper.rb

@@ -21,5 +21,9 @@ module Gitlab
     def gitlab_markdown?(filename)
       filename.downcase.end_with?(*%w(.mdown .md .markdown))
     end
+
+    def previewable?(filename)
+      gitlab_markdown?(filename) || markup?(filename)
+    end
   end
 end