After you browse to the file and then click "Add File" it does take a little while to upload ( you will see a blue progress bar at the bottom ). Until the the name then appears at the top of the file manager box it hasn't finished uploading ( and I am not talking about the file name at the bottom that came up when you browsed ). Then you can click "Done".

Because if you don't wait and keep trying to do it over and over again it thinks you are trying to upload multiple files and you start getting those error messages. Depending on the file size it may take a couple of minutes to upload.