def thumbnail_candidates
      [
        {
          file_collection: 'image_files',
          attribute: 'thumbnail_image_id'
        },
        {
          file_collection: 'image_files',
          attribute: 'background_image_id',
          unless: {
            attribute: 'background_type',
            value: 'video'
          }
        },
        {
          file_collection: 'image_files',
          attribute: 'poster_image_id',
          if: {
            attribute: 'background_type',
            value: 'video'
          }
        },
        {
          file_collection: 'video_files',
          attribute: 'video_file_id',
          if: {
            attribute: 'background_type',
            value: 'video'
          }
        }
      ]
    end
def thumbnail_candidates
      [
        {
          file_collection: 'image_files',
          attribute: 'thumbnail_image_id'
        },
        {
          file_collection: 'image_files',
          attribute: 'background_image_id',
          unless: {
            attribute: 'background_type',
            value: 'video'
          }
        },
        {
          file_collection: 'image_files',
          attribute: 'poster_image_id',
          if: {
            attribute: 'background_type',
            value: 'video'
          }
        },
        {
          file_collection: 'video_files',
          attribute: 'video_file_id',
          if: {
            attribute: 'background_type',
            value: 'video'
          }
        }
      ]
    end
A list of hashes used to determine a thumbnail for a page. Each
 hash in the list must contain two keys: `attribute` and
 `file_collection`.

 For each item, the given attribute is fetched from the page
 configuration and used to find a file from the given
 collection. `file_collection` must equal the collection_name of
 a registered {FileType}. The first file found is used as
 thumbnail.

     [
       {attribute: 'thumbnail_image_id', file_collection: 'image_files'},
       {attribute: 'background_image_id', file_collection: 'image_files'}
     ]

 It is possible to specify further conditions under which the
 candidate may be used via `if` or `unless` keys:

     [
       {
         attribute: 'video_id',
         file_collection: 'video_files',
         if: {
           attribute: 'background_type',
           value: 'video',
         }
       },
       {
         attribute: 'image_id',
         file_collection: 'image_files',
         unless: {
           attribute: 'background_type',
           value: 'video',
         }
       }
     ]

 The candidate will only be used if the given attribute has the
 given value.

 @returns {Array<Hash>}