Have one function for the form (which you already have) and then have a separate method for sending the form back.

In this case I have a function called FileUploadForm which constructs a form and returns it in a normal manner. The function ajax_file_form() then calls that function and returns the form as a string with the $form->forTemplate() method.
If you want a different template for the actual form then use $form->setTemplate() to set a new template. You have to have a function (as far as I know) that returns a new Form for the form handling to work properly. If I'd wanted to just print that form in a template I'd write $FileUploadForm to show it.

Hi,

it looks like the normal way to handle a form does not work when it is called using ajax? I have in my pageController:

function showProductDetails($action) {
    $productNumber = Director::urlParam("OtherID");
    $referer = Director::urlParams();
    $Link = $referer["URLSegment"]."/".$referer["Action"];
    if($this->isAjax) {
      // Get product details
      $details = DataObject::get_one("Article","ID = $productNumber");
      // Get product contents
      $contents = DataObject::get("ArticleChilds","ArticleID = $productNumber AND stock > 0");
      if(!empty($contents)) {
       $map = $contents->map('ID','Name');
       $fields = new FieldSet(
          new DropdownField('ID', 'Maak een keuze:', $map)
       );
      } else {
       $fields = new FieldSet(
       );
      }

      $actions = new FieldSet(
         new FormAction('doSubmitNewItem','Submit')
      );

      $form = new Form(
         $this,
         'showProductDetails',
         $fields,
         $actions
         );
      return $form->forTemplate();
    } else {
      return array();
    }
   }

   function doSubmitNewItem($data,$form) {
    // Save the form data

    return;
   }

However this does not in any way I try below the // Save the form data, actually save the form data. Do I make use of incorrect references to the save-function?

Thank in advance,
Maurice