markup_helper: detect .mkd and .mkdn as markdown

CHANGELOG

@@ -56,6 +56,7 @@ v 7.14.0 (unreleased)
   - Include branch/tag name in archive file and directory name
   - Add dropzone upload progress
   - Add a label for merged branches on branches page (Florent Baldino)
+  - Detect .mkd and .mkdn files as markdown (Ben Boeckel)
 
 v 7.13.3
   - Fix bug causing Bitbucket importer to crash when OAuth application had been removed.

lib/gitlab/markup_helper.rb

@@ -21,7 +21,7 @@ module Gitlab
     #
     # Returns boolean
     def gitlab_markdown?(filename)
-      filename.downcase.end_with?(*%w(.mdown .md .markdown))
+      filename.downcase.end_with?(*%w(.mdown .mkd .mkdn .md .markdown))
     end
 
     # Public: Determines if the given filename has AsciiDoc extension.

spec/lib/gitlab/markup_helper_spec.rb

@@ -15,7 +15,7 @@ describe Gitlab::MarkupHelper do
   end
 
   describe '#gitlab_markdown?' do
-    %w(mdown md markdown).each do |type|
+    %w(mdown mkd mkdn md markdown).each do |type|
       it "returns true for #{type} files" do
         expect(Gitlab::MarkupHelper.gitlab_markdown?("README.#{type}")).to be_truthy
       end