framework/validators/FileValidator.php
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;
    }
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;
    }
Checks if given uploaded file have correct type (extension) according current validator settings.
@param UploadedFile $file
@return bool