There are two ways for a user to add an attachment:
Create a new attribute with the type attachment under Request Management>Request and then add it to the Request Window.
There is an action configured already (Or can be added in the process as needed) that allow the user to add an attachment. However, he will only be able to add this attachment after the creation of the request (Clicking on Apply or OK at the top of the screen). But you need to ensure that the action is added to the process and the user is given the permission to use this action.
Nope that's fine. Just add it to the window.
Keep in mind that when a user add an attachment to this attribute it will NOT be added to the collection and only one attachment can be added.