if (!function_exists('wfu_after_upload_handler')) {
	function wfu_after_upload_handler($changable_data, $additional_data) {
		$files = $additional_data["files"];
		if ( count($files) > 0 ) {
			$url = str_replace(ABSPATH, site_url()."/", $files[0]["filepath"]);
			$changable_data["js_script"] = 'document.getElementById("mr-field-element-500082339575").value = "'.$url.'";';
		}
		return $changable_data;
	}
	add_filter('wfu_after_upload', 'wfu_after_upload_handler', 10, 2);
}

Form page - http://buzzfixer.com/test-wp-file-upload