yii\validators\FileValidator::validateExtension PHP Method

    protected function validateExtension($file)
    {
        $extension = mb_strtolower($file->extension, 'UTF-8');
        if ($this->checkExtensionByMimeType) {
            $mimeType = FileHelper::getMimeType($file->tempName, null, false);
            if ($mimeType === null) {
                return false;
            }
            $extensionsByMimeType = FileHelper::getExtensionsByMimeType($mimeType);
            if (!in_array($extension, $extensionsByMimeType, true)) {
                return false;
            }
        }
        if (!in_array($extension, $this->extensions, true)) {
            return false;
        }
        return true;
    }