Supporting files can be added to the build at any time by navigating to the Test Coverage primary tab, and then to the Builds secondary tab. In the specific build's details window, click on the supporting files icon in the right side vertical menu. The area expands where you can drag and drop files, or browse your computer for files to attach. If there is a purple numbered box below the supporting files icon, that indicates the number of files already present.
Users will need view and edit permissions in the test coverage module to perform this action. Depending on the settings of a users project permissions, users may be able to view all project builds or just a subset of them. Project owners manage the permissions for project members.