Add new guide upload-file

1.  UploadedFile::getInstances($model, 'file'); sees only one file if not set ```'file[]'```

```php
<?= $form->field($model, 'file[]')->fileInput(['multiple' => '']) ?>
```

2. ```$form->errorSummary(..)``` not return all errors of file attribute, but only the last.
So I had to invent.

```php
if ($model->hasErrors()) {
    echo '<pre>';
    print_r($model->getErrors());
    echo '</pre>';
}
```

So it should be?
Fixing the issue #3292

This change fixes the issue #3292. The details were mentioned in the comments to the issue.
Update DefaultValueValidator.php

Allow the default value to be a closure
Update CHANGELOG.md
Update DefaultValueValidator.php
MS SQL Server fix for older versions than 2012 (alternative solution)

This alternative solution LIMIT, OFFSET problem SQL Server version older than 2012.
Improve `exist` validator to support array attributes
Refactoring
Refactoring
Update config.php
BaseVarDumper::export() added
Doc comments for `BaseVarDumper::export()` added
Issue #3232 added to CHANGELOG
Object handling at `BaseVarDumper::export()` reworked
Tests updated
Tests updated
Tests updated
ExistValidator count logic fix
CHANGELOG
`allowArray` introduced.