Do you mean a single backend plone request? I'm not sure thats possible. Maybe if the widget is modified to give a special request at the end of the end of the download once all the files have been uploaded? Currently it uploads each one as a seperate request.
Or do you mean a JS event to notify the on the client side that all the files have been uploaded?
Okay, but I need to have a count of a total number of files being uploaded Or on the last request, there should be a method to check the same. And currently, I don't see any of those checks but as you see, the product could only start functioning once it gets to know that all files are uploaded.