File result = thumbnailManager.getThumbnailFile(docMap);
        assertNull(result);
    }

    // Test getThumbnailFile with blank path
    @Test
    public void test_getThumbnailFile_blankPath() {
        Map<String, Object> docMap = new HashMap<>();
        // No _id field, so docid will be null

        File result = thumbnailManager.getThumbnailFile(docMap);
        assertNull(result);
    }

    /**
     * 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);

            // 404
            throw responseManager.new404("Thumbnail for " + form.docId + " is not found.");
        }

        final File thumbnailFile = thumbnailManager.getThumbnailFile(doc);
        if (thumbnailFile == null) {
            if (fessConfig.isThumbnailEnabled()) {
                thumbnailManager.offer(doc);
            }
            // 404