Added empty display styles for list of files widgets #2033

Kept os_files_link and os_files_link_icon.  Used core's teaser and full.  Added os_files_preview and os_files_thumnbnail.  I don't know that these are all necessary - they're just placeholders until we have specifications for what view modes to use.
+    'image' => array('os_files_link', 'os_files_link_icon', 'teaser', 'full', 'os_files_preview', 'os_files_thumbnail'),