While in the trip planner, you will see the "Files" tab to the right of the "Notes" tab in the upper left bar.
From this tab, you can see all the files you uploaded, and you are able to search for files using the search bar in the top left.
If you have not yet added any files, you can upload a file using the "Upload" button in the top right. You can then select whichever file you want to add to your itinerary (ex. Boarding passes, hostel confirmation, activity tickets, etc).
You can also click the three dots on the right most side of the file and choose to Rename or Delete them.
If you click on the middle area next to a file under the "Attached To" section, you will be able to select a Transportation, Accommodation, or Activity item that you would like to add the file to in your itinerary. This will help you stay organized and keep all your files attached to the relevant item!