This articles gives you the guidelines of creatives that can be uploaded.
To upload a creative, click on the "Upload Creative" button that's on the Creatives tab.
This opens a modal window with the details of the creative you want to upload (see screenshots below).
Use the "Select campaign" field at the top to determine for which campaign the creative will be available.
You can either upload display creatives or video creatives.
Here are the available formats for display creatives:
Here are the available formats for video creatives: