@bergente
finally got some time to dive into it and you know it can be achieved but it's not possible to define indivually for every attachment. So to globally open the attachments in a new window you can do the following.
Go to:
Code:
/components/com_jem/common/views/tmpl/default_attachments.php
then find:
Code:
$attribs = array('class'=>'file-name');
and change it to
Code:
$attribs = array('class'=>'file-name','target'=>'_blank');
keep in mind that with an update of jem this change will be overwritten so better to make use of template overrides.