How do I add files to an event?

On the Calendar, navigate to the event on which you would like to add a file(s).
Once on the event, click the Upload Files button in the top-right.
Within the Upload Files modal which pops up, either drag and drop files directly or click the verbiage link to load files from your computer.
Once all desired files have loaded, click the Close button in the lower-right of the Upload Files modal.
You will now see the files that you have loaded listed in the right within the event.
To remove any of the loaded files, click the X to the right of the desired file.