By uploading the files individually, we were able to add an event handler to monitor the progress of the files being uploaded. This also opens up of the possibility of aborting the upload of individual files.
For this challenge why not see if you can add a mechanism for canceling the upload of a file. We already have the remove button which is used to remove files before they are uploaded. These could easily be updated so that they cancelled the upload if the upload is in progress.
A handler for the abort event can be added to the XHR object in the same way as the progress event handler was attached, so it should be easily achievable.