Hi Yossis,
You need to create custom component to display attachment if user is registered user. Please follow the below code which can be inserted into the forum message quilt:-
<#assign AttachedFile = rest("/messages/id/${env.context.message.id}/uploads/attachments").attachments.attachment />
<#list AttachedFile as file>
<#if !user.anonymous>
<a class="lia-link-navigation attachment-link" href="${file.url}">${file.title} ‏${(file.content.size?number/1024)?round} KB</a>
<#else>
<a class="lia-link-navigation attachment-link" href="/t5/user/userloginpage">${file.title} ‏${(file.content.size?number/1024)?round} KB</a>
</#if>
</#list>
Regards,
Vishwajit Shinde.