Better distinction between *Add main file* and *Add attachment*

We told testers to click on Add a File, and provided a screenshot for them to distinguish which Add a File they needed to click on. Half of them still got it wrong.

Since these two Add a File links do different things, shouldn't we distinguish between them? Like Add a main file and Add an attachment?

We should prevent the users to error, and make sure they know what they are attaching. A more clear message/label can avoid some unwanted doubts.

Proposed Solution
A - Add file
B - Add attachment