Look on this page for 'Upload File' (it's hard to miss, Mantis bug tracker can be a bit confusing). However, I can't reproduce -- works fine for me, so I still suspect it is something in your environment.
A screenshot of the error console, and confirmation you've emptied your cache, and that the browser is not in compatibility mode, would help more than a screenshot of the missing tree.
A screenshot of the error console, and confirmation you've emptied your cache, and that the browser is not in compatibility mode, would help more than a screenshot of the missing tree.