code: public static function getInstance($model, $attribute)
      {
          $name = Html::getInputName($model, $attribute);
          return static::getInstanceByName($name);
      }
docs: Returns an uploaded file for the given model attribute.
  The file should be uploaded using [[\yii\widgets\ActiveField::fileInput()]].
  @param \yii\base\Model $model the data model
  @param string $attribute the attribute name. The attribute name may contain array indexes.
  For example, '[1]file' for tabular file uploading; and 'file[1]' for an element in a file array.
  @return null|UploadedFile the instance of the uploaded file.
  Null is returned if no file is uploaded for the specified model attribute.
  @see getInstanceByName()