Pimcore\Model\Asset\Document::clearThumbnails PHP Method

    public function clearThumbnails($force = false)
    {
        if ($this->_dataChanged || $force) {
            // video thumbnails and image previews
            $files = glob(PIMCORE_TEMPORARY_DIRECTORY . "/document-image-cache/document_" . $this->getId() . "__*");
            if (is_array($files)) {
                foreach ($files as $file) {
                    unlink($file);
                }
            }
            recursiveDelete($this->getImageThumbnailSavePath());
        }
    }