Thanks for the idea. Basically the idea behind the upload control functionality was to provide an easier way to add some control to the toolbox. If you want to add a more complicated control with additional related files, we strongly suggest to do this from the file manager.
Adding related files for a control is a little bit complicated task due to the fact that the files could be related to one another (for example .js file could have a reference to some image files) and the system does not know how to upload them. That is why it is much easier and reliable to do this operation from the file manager.