Hi,
You can count the number of files then check in the loop if reach the last file and remove the separator. Just like
$files = rwmb_meta( 'planimetrie' );
$count = count( $files );
$i = 1;
foreach ( $files as $file ) {
if( $i < $count ) {
?>
<a href="<?php echo $file['url']; ?>" target="_blank"><?php echo $file['name']; ?></a>,
<?php
} else {
?>
<a href="<?php echo $file['url']; ?>" target="_blank"><?php echo $file['name']; ?></a>
<?php
}
$i++;
}