I can’t attach a file to my new ticket. Why ?

Since a user can update a ticket, he is able to insert an attached file.
So this is not a question of habilitation.

In fact, this functionality do not appear during creation.
You just have to save first to be able to attach a file.
This rule is valid for all depending items : attachments, notes, assignments, predecessor elements, successor elements and links between Risk, Actions and Issues.

Just one word : "save" first.