I have some lists on my SharePoint site that use custom new and edit forms. I found that these custom forms would not allow attachments. Implementing the Microsoft solution from here has solved the problem list 1 but not list 2.
I read somewhere that something that often helps is to go to the original list form web part, right click, web part properties and tick hidden (closed is already ticked). I did this on list 1 but on list 2 when I click web part properties it brings up the small window and displays the form as it would appear in a browser. Could this have something to do with it?