The reason why click coordinates does not work in the windows file manager is that ATS, which uses Selenium under the hood, only has control over the browser window. In other words, it is not possible to automate anything outside the browser window, such as the Windows File Manager.
That does not mean you can not test, however. Now, I am guessing that you are not trying to test the actual dropzone widget, correct?
In that case, I would recommend that you add a special button that is only visible to a tester that allows the file upload to be done in another way, for example by always creating a dummy file in a microflow. This will let you test what you really want to test without fighting with the dropzone widget.