If this is a file that will be distributed to attendees during the session, you can upload them through the session details. Just click Advanced Settings > + Manage Document
You can also add it through the Virtual Event Hub. Click My Speaking Schedule > + Add Document. If My Speaking Schedule is not appearing for you, you may need to add yourself as a speaker.
If this is a file that will be presented on screen, it cannot be uploaded, the speaker or the moderator will have to share their screen so that attendees can see the presentation. From the Studio, they will have to click the Share icon. Then select the share screen option, then select the file or the screen they want to show.