/**
     * Gets the thumbnail file for a document if it exists.
     *
     * @param docMap the document data map
     * @return the thumbnail file or null if not found
     */
    public File getThumbnailFile(final Map<String, Object> docMap) {
        final String thumbnailPath = getImageFilename(docMap);
        if (StringUtil.isNotBlank(thumbnailPath)) {
            final File file = new File(baseDir, thumbnailPath);