- Enh #3636: Hide menu container tag with empty items in `yii\widgets\Menu` (arturf)
- Enh #3643: Improved Mime-Type detection by using the `mime.types` file from apache http project to dected mime types by file extension (cebe, pavel-voronin, trejder)
- Enh #3773: Added `FileValidator::mimeTypes` to support validating MIME types of files (Ragazzo)
- Enh #3774: Added `FileValidator::checkTypeAgainstMime` to support validating file types against file mime-types (Ragazzo)
- Enh #3774: Added `FileValidator::checkExtensionByMimeType` to support validating file types against file mime-types (Ragazzo)
- Enh: Added support for using sub-queries when building a DB query with `IN` condition (qiangxue)
- Enh: Supported adding a new response formatter without the need to reconfigure existing formatters (qiangxue)
- Enh: Added `yii\web\UrlManager::addRules()` to simplify adding new URL rules (qiangxue)
