Add user guidance for OpenDocument attachments

https://www.pivotaltracker.com/story/show/72066166

  As a user looking at a publication with an attached document published in an OpenDocument format (ODS, ODP, ODT) I want to know what the document is and what software exists to allow to me view/edit it. 
Since these constants should be immutable, this could be benefit from being frozen: ``` OPENDOCUMENT_EXTENSIONS = %w(ODT ODP ODF).freeze ``` (eg like Edition model (edition.rb, lines 41 to 44))
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