CFileValidator

protected function emptyAttribute($object, $attribute)
{
    if(!$this->allowEmpty)
    {
        $message=$this->message!==null?$this->message : Yii::t('yii','{attribute} cannot be blank.');
        $this->addError($object,$attribute,$message);
    }
}

protected function getSizeLimit()
{
    $limit=ini_get('upload_max_filesize');
    $limit=$this->sizeToBytes($limit);
    if($this->maxSize!==null && $limit>0 && $this->maxSize<$limit)
        $limit=$this->maxSize;
    if(isset($_POST['MAX_FILE_SIZE']) && $_POST['MAX_FILE_SIZE']>0 && $_POST['MAX_FILE_SIZE']<$limit)
        $limit=$_POST['MAX_FILE_SIZE'];
    return $limit;
}

protected function validateAttribute($object, $attribute)
{
    if($this->maxFiles > 1)
    {
        $files=$object->$attribute;
        if(!is_array($files) || !isset($files[0]) || !$files[0] instanceof CUploadedFile)
            $files = CUploadedFile::getInstances($object, $attribute);
        if(array()===$files)
            return $this->emptyAttribute($object, $attribute);
        if(count($files) > $this->maxFiles)
        {
            $message=$this->tooMany!==null?$this->tooMany : Yii::t('yii', '{attribute} cannot accept more than {limit} files.');
            $this->addError($object, $attribute, $message, array('{attribute}'=>$attribute, '{limit}'=>$this->maxFiles));
        }
        else
            foreach($files as $file)
                $this->validateFile($object, $attribute, $file);
    }
    else
    {
        $file = $object->$attribute;
        if(!$file instanceof CUploadedFile)
        {
            $file = CUploadedFile::getInstance($object, $attribute);
            if(null===$file)
                return $this->emptyAttribute($object, $attribute);
        }
        $this->validateFile($object, $attribute, $file);
    }
}

protected function validateFile($object, $attribute, $file)
{
    if(null===$file || ($error=$file->getError())==UPLOAD_ERR_NO_FILE)
        return $this->emptyAttribute($object, $attribute);
    else if($error==UPLOAD_ERR_INI_SIZE || $error==UPLOAD_ERR_FORM_SIZE || $this->maxSize!==null && $file->getSize()>$this->maxSize)
    {
        $message=$this->tooLarge!==null?$this->tooLarge : Yii::t('yii','The file "{file}" is too large. Its size cannot exceed {limit} bytes.');
        $this->addError($object,$attribute,$message,array('{file}'=>$file->getName(), '{limit}'=>$this->getSizeLimit()));
    }
    else if($error==UPLOAD_ERR_PARTIAL)
        throw new CException(Yii::t('yii','The file "{file}" was only partially uploaded.',array('{file}'=>$file->getName())));
    else if($error==UPLOAD_ERR_NO_TMP_DIR)
        throw new CException(Yii::t('yii','Missing the temporary folder to store the uploaded file "{file}".',array('{file}'=>$file->getName())));
    else if($error==UPLOAD_ERR_CANT_WRITE)
        throw new CException(Yii::t('yii','Failed to write the uploaded file "{file}" to disk.',array('{file}'=>$file->getName())));
    else if(defined('UPLOAD_ERR_EXTENSION') && $error==UPLOAD_ERR_EXTENSION)  // available for PHP 5.2.0 or above
        throw new CException(Yii::t('yii','File upload was stopped by extension.'));

    if($this->minSize!==null && $file->getSize()<$this->minSize)
    {
        $message=$this->tooSmall!==null?$this->tooSmall : Yii::t('yii','The file "{file}" is too small. Its size cannot be smaller than {limit} bytes.');
        $this->addError($object,$attribute,$message,array('{file}'=>$file->getName(), '{limit}'=>$this->minSize));
    }

    if($this->types!==null)
    {
        if(is_string($this->types))
            $types=preg_split('/[\s,]+/',strtolower($this->types),-1,PREG_SPLIT_NO_EMPTY);
        else
            $types=$this->types;
        if(!in_array(strtolower($file->getExtensionName()),$types))
        {
            $message=$this->wrongType!==null?$this->wrongType : Yii::t('yii','The file "{file}" cannot be uploaded. Only files with these extensions are allowed: {extensions}.');
            $this->addError($object,$attribute,$message,array('{file}'=>$file->getName(), '{extensions}'=>implode(', ',$types)));
        }
    }
}