Add new guide upload-file

docs/guide/upload-file.md

+# Upload File with Yii2
+
+### First you need to create a model that will handle the form of download the file.
+
+```php
+namespace app\models;
+
+use yii\base\Model;
+use yii\web\UploadedFile;
+
+/**
+ * UploadForm is the model behind the upload form.
+ */
+class UploadForm extends Model
+{
+    /**
+     * @var UploadedFile|Null file attribute
+     */
+    public $file;
+
+    /**
+     * @return array the validation rules.
+     */
+    public function rules()
+    {
+        return [
+            [['file'], 'file'],
+        ];
+    }
+}
+```
+In this code, we created a model ```UploadForm``` with an attribute ```$file``` that will be is ```<input type="file">``` in upload form and pointed out to him validation rule ```file```. This rule is [[yii\validators\FileValidator|FileValidator]]
+
+### Secondly create a view for our model.
+```php
+<?php
+use yii\widgets\ActiveForm;
+
+$form = ActiveForm::begin(['options' => ['enctype' => 'multipart/form-data']]); ?>
+
+<?= $form->field($model, 'file')->fileInput() ?>
+
+<button>Submit</button>
+
+<?php ActiveForm::end(); ?>
+```
+
+It is different attribute ```'enctype' => 'multipart/form-data'``` from the standard form. This value is required when you are using forms that have a file upload control. ```fileInput()``` represents a form input field.
+
+### Thirdly, that create the controller that will connect our form and model.
+```php
+namespace app\controllers;
+
+use Yii;
+use yii\web\Controller;
+use app\models\UploadForm;
+use yii\web\UploadedFile;
+
+class SiteController extends Controller
+{
+    public function actionUpload()
+    {
+        $model = new UploadForm();
+
+        if (Yii::$app->request->isPost) {
+            $model->file = UploadedFile::getInstance($model, 'file');
+
+            if ($model->validate()) {                
+                $model->file->saveAs('uploads/' . $model->file->baseName . '.' . $model->file->extension);
+            }
+        }
+
+        return $this->render('upload', ['model' => $model]);
+    }
+}
+```
+The difference here from the standard crud action, so use ```UploadedFile::getInstance(...)``` instead ```model->load(...)```. [[\yii\web\UploadedFile|UploadedFile]] does not run the model validation, it only provides information about the uploaded file. Therefore, you need to run validation manually ```$model->validate()```. This triggers the [[yii\validators\FileValidator|FileValidator]] that expects a file
+```php
+$file instanceof UploadedFile || $file->error == UPLOAD_ERR_NO_FILE //in code framework
+```
+
+If validation done without errors, then save the file 
+```php
+$model->file->saveAs('uploads/' . $model->file->baseName . '.' . $model->file->extension);
+```
+If you use "basic" application then forlder ```uploads``` should be create inside ```web``` folder.
+
+Everything is ready, now run the page and download the file. Check the folder ```basic/web/uploads``` to make sure that you have downloaded.
+
+## Additional information.
+
+***
+
+### Required rule
+
+If you need to check the mandatory download the file, then use ```skipOnEmpty```.
+```php
+public function rules()
+{
+    return [
+        [['file'], 'file', 'skipOnEmpty' => false],
+    ];
+}
+```
+
+***
+
+### Path upload folder
+
+Folder to download the file can be installed using ```Yii::getAlias('@app/uploads')```. This base path of currently running application and folder ```uploads``
+
+***
+
+### MIME type
+
+FileValidator have property ```$types```
+```php
+public function rules()
+{
+    return [
+        [['file'], 'file', 'types' => 'gif, jpg',],
+    ];
+}
+```
+it pulls
+```php
+in_array(strtolower(pathinfo($file->name, PATHINFO_EXTENSION)), $this->types, true))
+```
+As you can see, the name of the expansion may be one and the file type - other, actually.
+
+``UploadedFile::getInstance()->type``` also do not take this value for granted.
+Instead, use [[\yii\helpers\BaseFileHelper|FileHelper]] and his [[FileHelper::getMimeType()]] to determine the exact MIME type. 
+
+If allowed to **load only the images**, using [[\yii\validators\ImageValidator|ImageValidator]] instead  [[yii\validators\FileValidator|FileValidator]].
+
+```php
+public function rules()
+{
+    return [
+        [['file'], 'image', 'mimeTypes' => 'image/jpeg, image/png',],
+    ];
+}
+```
+```ImageValidator``` use use ```yii\helpers\FileHelper;``` for check mime types. 
+[List Mime types](http://en.wikipedia.org/wiki/Internet_media_type#List_of_common_media_types)
+
+***
+
+### Multiple files uploader
+
+If you need download multiple files, you will need to alter slightly the controller and view.
+At first view:
+
+```php
+<?php
+use yii\widgets\ActiveForm;
+
+$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' => '']) ?>
+
+    <button>Submit</button>
+
+<?php ActiveForm::end(); ?>
+```
+
+In fact the only difference is in the one row.
+```php
+<?= $form->field($model, 'file[]')->fileInput(['multiple' => '']) ?>
+```
+instead
+```php
+<?= $form->field($model, 'file')->fileInput() ?>
+```
+
+* ```['multiple' => '']``` - HTML <input> multiple Attribute
+* ```file[]``` vs ```file`` - need, otherwise UploadedFile sees only one file
+
+We now turn to the controller
+```php
+namespace app\controllers;
+
+use Yii;
+use yii\web\Controller;
+use app\models\UploadForm;
+use yii\web\UploadedFile;
+
+class SiteController extends Controller
+{
+    public function actionUpload()
+    {
+        $model = new UploadForm();
+
+        if (Yii::$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', $error);
+                    }
+                }
+            }
+
+            if ($model->hasErrors('file')){
+                $model->addError(
+                    'file',
+                    count($model->getErrors('file')) . ' of ' . count($files) . ' files not uploaded'
+                );
+            }
+
+        }
+
+        return $this->render('upload', ['model' => $model]);
+    }
+}
+```
+Here the differences in:
+* ``` UploadedFile::getInstances($model, 'file');``` instead ``` UploadedFile::getInstance($model, 'file');```. First returns **all** uploaded files for the given model attribute, second - one.
+* All other differences follow from the first.