String title = DocumentUtil.getValue(document, fessConfig.getIndexFieldTitle(), String.class);
        if (StringUtil.isBlank(title)) {
            title = DocumentUtil.getValue(document, fessConfig.getIndexFieldFilename(), String.class);
            if (StringUtil.isBlank(title)) {
                title = DocumentUtil.getValue(document, fessConfig.getIndexFieldUrl(), String.class);
            }
        }

     * The value is, e.g. filename <br>
     * comment: Field name for file name in the index.
     * @return The value of found property. (NotNull: if not found, exception but basically no way)
     */
    String getIndexFieldFilename();

    /**
     * Get the value for the key 'index.field.thumbnail'. <br>
     * The value is, e.g. thumbnail <br>
     * comment: Field name for thumbnail in the index.