How to add a javascript field to custom content type
First things first: a warning. 
This sounds like an extremely dangerous idea. If you do this, please at least make sure that you're only opening this up to trusted users (as allowing people to place javascript on your page could lead to some very, very bad things, much like the php filter.)

That being said, if your sure you want to do this, your gonna need to add the js to your page programmaticly. You could easily do this through your theme functions of template_preprocess_node.

Find that function in your template.php file. You'll want to add a condition to check for the content type, then grab the js file and use drupal_add_js to grab the attached js file and attach it to the head.