yii\validators\FileValidator::validateValue PHP Method

    protected function validateValue($value)
    {
        if (!$value instanceof UploadedFile || $value->error == UPLOAD_ERR_NO_FILE) {
            return [$this->uploadRequired, []];
        }
        switch ($value->error) {
            case UPLOAD_ERR_OK:
                if ($this->maxSize !== null && $value->size > $this->getSizeLimit()) {
                    return [$this->tooBig, ['file' => $value->name, 'limit' => $this->getSizeLimit(), 'formattedLimit' => Yii::$app->formatter->asShortSize($this->getSizeLimit())]];
                } elseif ($this->minSize !== null && $value->size < $this->minSize) {
                    return [$this->tooSmall, ['file' => $value->name, 'limit' => $this->minSize, 'formattedLimit' => Yii::$app->formatter->asShortSize($this->minSize)]];
                } elseif (!empty($this->extensions) && !$this->validateExtension($value)) {
                    return [$this->wrongExtension, ['file' => $value->name, 'extensions' => implode(', ', $this->extensions)]];
                } elseif (!empty($this->mimeTypes) && !$this->validateMimeType($value)) {
                    return [$this->wrongMimeType, ['file' => $value->name, 'mimeTypes' => implode(', ', $this->mimeTypes)]];
                }
                return null;
            case UPLOAD_ERR_INI_SIZE:
            case UPLOAD_ERR_FORM_SIZE:
                return [$this->tooBig, ['file' => $value->name, 'limit' => $this->getSizeLimit(), 'formattedLimit' => Yii::$app->formatter->asShortSize($this->getSizeLimit())]];
            case UPLOAD_ERR_PARTIAL:
                Yii::warning('File was only partially uploaded: ' . $value->name, __METHOD__);
                break;
            case UPLOAD_ERR_NO_TMP_DIR:
                Yii::warning('Missing the temporary folder to store the uploaded file: ' . $value->name, __METHOD__);
                break;
            case UPLOAD_ERR_CANT_WRITE:
                Yii::warning('Failed to write the uploaded file to disk: ' . $value->name, __METHOD__);
                break;
            case UPLOAD_ERR_EXTENSION:
                Yii::warning('File upload was stopped by some PHP extension: ' . $value->name, __METHOD__);
                break;
            default:
                break;
        }
        return [$this->message, []];
    }

 /**
  * @inheritdoc
  */
 protected function validateValue($file)
 {
     $res = parent::validateValue($file);
     if (empty($res)) {
         return $this->_validateImage($file);
     }
     return $res;
 }