Prompt page leave if unsubmitted upload

[juniwalk]

Hi,
I am trying to implement function that prompts confirmation on page close if there are unsubmitted uploaded files. This works alright if I use the [Browse] button, however if I drag and drop files onto upload area form is not detected as being "dirty" and no prompt is shown.

Could you add some hidden input which would get value updated (to anything) on drag and drop upload which would make this possible?