defaultuser

I just registered to answer this as I was having the same problem and found the solution, so I thought it might help others in the future to document it. For some reason (bug?) it seems you can't select the files directly if you click on Add Files, but if you click on Add Folder and select the folder the files are in, it lets you select them individually. Alternatively, you can click on Open and drag-and-drop the file into Switch from the Finder window.