Adds an index to attachments.disk_filename (#25022).

Patch by Jens Kraemer.

git-svn-id: http://svn.redmine.org/redmine/trunk@16352 e93f8b46-1217-0410-a6f0-8f06a7374b81

db/migrate/20170207050700_add_index_on_disk_filename_to_attachments.rb

@@ -0,0 +1,5 @@
+class AddIndexOnDiskFilenameToAttachments < ActiveRecord::Migration
+  def change
+    add_index :attachments, :disk_filename
+  end
+end