The Dropzone component introduces:

The method <%= code("render_dropzone") %> defined in <%= code("app/helpers/components/dropzone_helper.rb") %> will render the dropzone widget that is bound to the Stimulus controller. Upon triggering the file dialog or dragging and dropping files, the controller will delegate the files individually to a function, <%= code("uploadFile") %>. It is up to you to define what to do when a a file is to be uploaded.

To implement fully, edit the <%= code("uploadFile") %> function in <%= code("app/javascript/controllers/ui/dropzone_controller.js") %>. This function is called for each file that is to be uploaded. The function is passed the file object.

If you try the example component, you will see that it is logging the file object to the console.