def get_existing_thumbnail(self, thumbnail_options, high_resolution=False):
        """
        Return a ``ThumbnailFile`` containing an existing thumbnail for a set
        of thumbnail options, or ``None`` if not found.
        """
        thumbnail_options = self.get_options(thumbnail_options)
        names = [
            self.get_thumbnail_name(
                thumbnail_options, transparent=False,
                high_resolution=high_resolution)]
        transparent_name = self.get_thumbnail_name(
            thumbnail_options, transparent=True,
            high_resolution=high_resolution)
        if transparent_name not in names:
            names.append(transparent_name)

        for filename in names:
            exists = self.thumbnail_exists(filename)
            if exists:
                thumbnail_file = ThumbnailFile(
                    name=filename, storage=self.thumbnail_storage,
                    thumbnail_options=thumbnail_options)
                if settings.THUMBNAIL_CACHE_DIMENSIONS:
                    # If this wasn't local storage, exists will be a thumbnail
                    # instance so we can store the image dimensions now to save
                    # a future potential query.
                    thumbnail_file.set_image_dimensions(exists)
                return thumbnail_file
Return a ``ThumbnailFile`` containing an existing thumbnail for a set
        of thumbnail options, or ``None`` if not found.