Ok, so you have a HTML 5 file input markup on your view and you have enabled multiple attribute to true. You may also be using widgets based on HTML5 input like \kartik\widgets\FileInput. But when you read a file in Yii controller using CUploadedFile (Yii1) or UploadedFile (Yii2), you do not see a list of multiple files that you selected in your view, but only one row. Your HTML markup in your view would be something like this and you don't know why this is not working?
With the adoption of PJax on Yii2 things have change quite a bit with GridView when it comes to work with them in AJAX mode. It will probably be confusing at the beginning, but then you will soon realize how powerful the new approach is.