Fixed not really correct file uploading in the guide

docs/guide/input-file-upload.md

 Uploading Files
 ===============
 
-> Note: This section is under development.
+Uploading files in Yii is done via the a form model, its validation rules and some controller code. Let's review what's
+required to handle uploads properly.
 
-Uploading files in Yii is done via the a form model, its validation rules and some controller code. Let's review what's needed
-to handle uploads properly.
 
-Form model
-----------
+Uploading single file
+---------------------
 
 First of all, you need to create a model that will handle file uploads. Create `models/UploadForm.php` with the following
 content:
@@ -24,7 +23,7 @@ use yii\web\UploadedFile;
 class UploadForm extends Model
 {
     /**
-     * @var UploadedFile|Null file attribute
+     * @var UploadedFile file attribute
      */
     public $file;
 
@@ -40,11 +39,10 @@ class UploadForm extends Model
 }
 ```
 
-In the code above, we created a model `UploadForm` with an attribute `$file` that will become `<input type="file">` in
+In the code above, we've created a model `UploadForm` with an attribute `$file` that will become `<input type="file">` in
 the HTML form. The attribute has the validation rule named `file` that uses [[yii\validators\FileValidator|FileValidator]].
 
-Form view
----------
+### Form view
 
 Next, create a view that will render the form:
 
@@ -65,8 +63,7 @@ use yii\widgets\ActiveForm;
 The `'enctype' => 'multipart/form-data'` is necessary because it allows file uploads. `fileInput()` represents a form
 input field.
 
-Controller
-----------
+### Controller
 
 Now create the controller that connects the form and model together:
 
@@ -114,10 +111,13 @@ If you're using the "basic" application template, then folder `uploads` should b
 
 That's it. Load the page and try uploading. Uploads should end up in `basic/web/uploads`.
 
-Additional information
-----------------------
+Validation
+----------
+
+It's often required to adjust validation rules to accept certain files only or require uploading. Below we'll review
+some common rule configurations.
 
-### Required rule
+### Required
 
 If you need to make the file upload mandatory, use `skipOnEmpty` like the following:
 
@@ -156,12 +156,13 @@ public function rules()
 
 [List of common media types](http://en.wikipedia.org/wiki/Internet_media_type#List_of_common_media_types)
 
-### Validating uploaded image
+### Image properties
 
 If you upload an image, [[yii\validators\ImageValidator|ImageValidator]] may come in handy. It verifies if an attribute
 received a valid image that can be then either saved or processed using the [Imagine Extension](https://github.com/yiisoft/yii2/tree/master/extensions/imagine).
 
-### Uploading multiple files
+Uploading multiple files
+------------------------
 
 If you need to download multiple files at once, some adjustments are required.
  
@@ -194,15 +195,9 @@ View:
 use yii\widgets\ActiveForm;
 
 $form = ActiveForm::begin(['options' => ['enctype' => 'multipart/form-data']]);
-
-if ($model->hasErrors()) { //it is necessary to see all the errors for all the files.
-    echo '<pre>';
-    print_r($model->getErrors());
-    echo '</pre>';
-}
 ?>
 
-<?= $form->field($model, 'file[]')->fileInput(['multiple' => '']) ?>
+<?= $form->field($model, 'file[]')->fileInput(['multiple' => true]) ?>
 
     <button>Submit</button>
 
@@ -212,7 +207,7 @@ if ($model->hasErrors()) { //it is necessary to see all the errors for all the f
 The difference is the following line:
 
 ```php
-<?= $form->field($model, 'file[]')->fileInput(['multiple' => '']) ?>
+<?= $form->field($model, 'file[]')->fileInput(['multiple' => true]) ?>
 ```
 
 Controller:
@@ -232,31 +227,13 @@ class SiteController extends Controller
         $model = new UploadForm();
 
         if (Yii::$app->request->isPost) {
-
-            $files = UploadedFile::getInstances($model, 'file');
-
-            foreach ($files as $file) {
-
-                $_model = new UploadForm();
-
-                $_model->file = $file;
-
-                if ($_model->validate()) {
-                    $_model->file->saveAs('uploads/' . $_model->file->baseName . '.' . $_model->file->extension);
-                } else {
-                    foreach ($_model->getErrors('file') as $error) {
-                        $model->addError('file', $error);
-                    }
+            $model->file = UploadedFile::getInstances($model, 'file');
+            
+            if ($model->file && $model->validate()) {
+                foreach ($model->file as $file) {
+                    $file->saveAs('uploads/' . $file->baseName . '.' . $file->extension);
                 }
             }
-
-            if ($model->hasErrors('file')){
-                $model->addError(
-                    'file',
-                    count($model->getErrors('file')) . ' of ' . count($files) . ' files not uploaded'
-                );
-            }
-
         }
 
         return $this->render('upload', ['model' => $model]);
@@ -264,5 +241,6 @@ class SiteController extends Controller
 }
 ```
 
-The difference is using `UploadedFile::getInstances($model, 'file');` instead of `UploadedFile::getInstance($model, 'file');`.
-The former returns instances for **all** uploaded files while the latter gives you only a single instance.
+There are two differences from single file upload. First is that `UploadedFile::getInstances($model, 'file');` used
+instead of `UploadedFile::getInstance($model, 'file');`. The former returns instances for **all** uploaded files while
+the latter gives you only a single instance. The second difference is that we're doing `foreach` and saving each file.