You should be able to override this method in the Controller class using that form.
Could please give me a quick example? I'm not sure how to proceed with it.
What I suggested above won't work, the handler is prefixed with the Widget alias... one way that'll work is if you create a new FormWidget that extends the core FileUpload widget and override its onUpload() method. Then register this formwidget in your plugin and use the new name in your forms.
1-4 of 4