If you want to add a file, you will have to contact the Exposure It team via email to have them add it.
As of right now, there isn't a way for you to add it yourself, but this is something we are working to fix.
However, if you already have a file listed, you can change the name of the file by clicking on the pencil icon.
Note that there is not a file preview on the property site, just a button with the file name and the option to download it.