Possibly there's a bug in the Javascript and/or it doesn't work in the browser you have.
ok, then my question is: the normal button "add attachment" is not in javascript? instead the "add attachment" that comes out with ILA Mod is in javascript?
because i thought they were the same, i don't see difference between them (and the code too, it appears - to me- the same)... but i'm not a code good boy... 
They are different, ILA adds an additional function and has to keep track of the attachment number so things get assigned to the right spots. Anyway what we have is some javascript problems, oh joy!
What browser and version are you using? There must be a setting to display script errors or a addon that will do that. Knowing where its javascript engine is tossing a cog will be a big help in tracking this down.
Looking back through that code, the only odd thing I immediately see is in post.template.php ,
find:
var count_attachments = ', count($context['current_attachments']) ,' + 1;
replace with
var count_attachments = ', count($context['current_attachments']) + 1,';See if that makes any difference at all ... and
remember to put it back to the way it was or the MOD will not uninstall cleanly. So save a copy of the post.template.php before you start doing edits so you can get back to where you were.