Fixes #6589: updated code style guide to mention else after return, adjusted code involved in the issue

Works around PHP Bug https://bugs.php.net/bug.php?id=68528

fixes #5962

Fixes #5587: `json_encode` is now used with `JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE` where it makes sense, also it is now default for `Json::encode()`

Fixes #3581: Added `yii\validators\CompareValidator::type` to support type conversion before comparing values

Fixes #5039: `UniqueValidator` and `ExistValidator` did not respect query conditions added by default scope

I'm find error

```php

Wrong
 } elseif (!empty($this->mimeTypes) && !in_array(FileHelper::getMimeType($file->tempName), $this->mimeTypes, true)) {

Correct
} elseif (!empty($this->mimeTypes) && !in_array(FileHelper::getMimeType($file), $this->mimeTypes, true)) {
```

Not need call '$file->tempName' because getMimeType get object $_FILE => $file, so use $file and validator works fine