File::isForbidden() 

\nn\t3::File()->isForbidden($filename = NULL, $allowed = []); 

Indicates whether the file type is prohibited

\nn\t3::File()->isForbidden('image.jpg'); => returns 'false'
\nn\t3::File()->isForbidden('image.xyz', ['xyz']); => returns 'false'
\nn\t3::File()->isForbidden('hack.php'); => returns 'true'
\nn\t3::File()->isForbidden('.htaccess'); => returns 'true'
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@param string $filename
@param array $allowed
@return boolean

Source Code 

public function isForbidden($filename = null, $allowed = [])
{
	if (!$filename) return false;
	if (substr($filename, 0, 1) == '.') return true;
	if (!$allowed) {
		$types = array_values(self::$TYPES);
		$allowed = array_merge(...$types);
	}
	$suffix = $this->suffix($filename);
	return !in_array($suffix, $allowed);
}
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