Pimcore\Model\Asset\Image\Thumbnail::getFileExtension PHP Method

    public function getFileExtension()
    {
        $mapping = ["image/png" => "png", "image/jpeg" => "jpg", "image/gif" => "gif", "image/tiff" => "tif", "image/svg+xml" => "svg"];
        $mimeType = $this->getMimeType();
        if (isset($mapping[$mimeType])) {
            return $mapping[$mimeType];
        }
        if ($this->getAsset()) {
            return \Pimcore\File::getFileExtension($this->getAsset()->getFilename());
        }
        return "";
    }