Hello Community,

as i found nothing about this problem in the forum that worked for me, i want to post my solution.
Maybe this will help some newbies like me

I wanted do build a small Gallery with HasManyFileManager and needed something like this with floating elemts:
IMG margin IMG margin IMG margin IMG
IMG margin IMG margin IMG margin IMG

So i need to check evety nth item to put whatever in my template code.
As postet here by Sam, i can not access some iterator function placed in Page_Controller or Page class from within my <% control ... %>-function

This is how i finally did it:
I can now define the name/group and number of columns in my template (it's more flexible than defining this in my php file - maybe i will have multiple HasManyFileManager on a page or want different numbers or columns on different pages) and i don't lose the sorting of the images that i've done in HasManyFileManager:

GalleryPage.php

class GaleriePage extends Page {
......
	function getCMSFields() {
	......
		$minigallery = new HasManyFileManager(
		   $this,
		   'GalleryImages', // name -> will be used for file grouping
		   'Images' // relation name defined in $has_many
		);
	$fields->addFieldToTab('Root.Content.Galerie', $minigallery);
	......
	}

......

	function AttachedFilesColumns($Group,$Cols=0) {
		$Data = $this -> AttachedFiles($Group);
		$output = new DataObjectSet(); 	
		$i = 0;
		foreach($Data as $Data) {
			$i++;
			if($Cols != 0) {
				if($i % $Cols == 0) {
					$Data->Col = "last";
					$i = 0;
				} else {
					$Data->Col = $i;
				}
			} else {
				$Data->Col = $i;
			}			
			
			$output->push($Data);
		}
		return $output;
	}



}

now in my template instead of "<% control AttachedFiles(GalleryImages) %>" i can use "<% control AttachedFilesColumns(GalleryImages,4) %>" where argument1 is the group name used by HasManyFileManager and argument2 is the number of columns.

Example:

<% control AttachedFilesColumns(GalleryImages,4) %>
   <div class="GalerieThumb-$Col"><a>img<a></div>
<% end_control %>

Output:
<div class="GalerieThumb-1"><a>img<a></div>
<div class="GalerieThumb-2"><a>img<a></div>
<div class="GalerieThumb-3"><a>img<a></div>
<div class="GalerieThumb-last"><a>img<a></div>
<div class="GalerieThumb-1"><a>img<a></div>
<div class="GalerieThumb-2"><a>img<a></div>
<div class="GalerieThumb-3"><a>img<a></div>
<div class="GalerieThumb-last"><a>img<a></div>
....

now the last "column" in "each row" is accessible via css
(or i could use an if-block to check the $Col-output and place tr-tags for a table in the code)

-----------------------

wihtout argument2 for number of columns

<% control AttachedFilesColumns(GalleryImages) %>
   <div class="GalerieThumb-$Col"><a>img<a></div>
<% end_control %>

Output:

<div class="GalerieThumb-1"><a>img<a></div>
<div class="GalerieThumb-2"><a>img<a></div>
<div class="GalerieThumb-3"><a>img<a></div>
<div class="GalerieThumb-4"><a>img<a></div>
<div class="GalerieThumb-5"><a>img<a></div>
<div class="GalerieThumb-6"><a>img<a></div>