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).
To delete a file, you can click the three dots next on the right most side of the file and select "Delete".