Add new thumbnail icons for Word and Excel files

Also covers OpenOffice formats and 'x' variants.

https://www.pivotaltracker.com/story/show/38788757
Fix broken tests and fixture file
Merge pull request #664 from alphagov/new-excel-and-word-thumbnails

Add new thumbnail icons for Word and Excel files

https://www.pivotaltracker.com/story/show/38788757