The default path of thumbnails is ".tmb". If you want to change the path, need to change the value of "tmbPath" in connector.php:

$opts = array(
	// 'debug' => true,
	'roots' => array(
		array(
			'driver'        => 'LocalFileSystem',   // driver for accessing file system (REQUIRED)
			'path'          => '../../../../../uploads/',         // path to files (REQUIRED)
			'URL'           => dirname($_SERVER['PHP_SELF']) . '/../../../../../uploads/', // URL to files (REQUIRED)
			'tmbPath'    => 'thumbnails', 
			'accessControl' => 'access',             // disable and hide dot starting files (OPTIONAL)
			'attributes' => array(
		        array(
		            'pattern' => '!^/*.*$!',
		            'read' => true,
		            'write' => true,
		            'locked' => false
		        )
		    ),
		)
	)
);

Another reason that thumbnails don't show would be the permission issue. Make sure the permission of upload folder is 777.

Reference:

• https://github.com/Studio-42/elFinder/wiki/Connector-configuration-options