Merge pull request #6543 from softark/docs-guide-input-file-upload-revised

docs/guide/input-file-upload.md

 Uploading Files
 ===============
 
-Uploading files in Yii is done via the a form model, its validation rules and some controller code. Let's review what's
+Uploading files in Yii is done via a form model, its validation rules and some controller code. Let's review what's
 required to handle uploads properly.
 
 
@@ -39,7 +39,7 @@ class UploadForm extends Model
 }
 ```
 
-In the code above, we've 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
@@ -65,7 +65,7 @@ input field.
 
 ### Controller
 
-Now create the controller that connects the form and model together:
+Now create the controller that connects the form and the model together:
 
 ```php
 namespace app\controllers;
@@ -95,13 +95,16 @@ class SiteController extends Controller
 ```
 
 Instead of `model->load(...)`, we are using `UploadedFile::getInstance(...)`. [[\yii\web\UploadedFile|UploadedFile]] 
-does not run the model validation, rather it only provides information about the uploaded file. Therefore, you need to run the validation manually via `$model->validate()` to trigger the [[yii\validators\FileValidator|FileValidator]] that expects a file:
+does not run the model validation, rather it only provides information about the uploaded file. Therefore, you need to run the validation manually via `$model->validate()` to trigger the [[yii\validators\FileValidator|FileValidator]]. The validator expects that
+the attribute is an uploaded file, as you see in the core framework code:
 
 ```php
-$file instanceof UploadedFile || $file->error == UPLOAD_ERR_NO_FILE //in the code framework
+if (!$file instanceof UploadedFile || $file->error == UPLOAD_ERR_NO_FILE) {
+    return [$this->uploadRequired, []];
+}
 ```
 
-If validation is successful, then we're saving the file: 
+If the validation is successful, then we're saving the file: 
 
 ```php
 $model->file->saveAs('uploads/' . $model->file->baseName . '.' . $model->file->extension);
@@ -241,6 +244,6 @@ class SiteController extends Controller
 }
 ```
 
-There are two differences from single file upload. First is that `UploadedFile::getInstances($model, 'file');` used
+There are two differences from single file upload. First is that `UploadedFile::getInstances($model, 'file');` is 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.