So, show us the code they have written for each.
Webdriver cannot do anything outside the browser window, so I think you need to show us the specific developer code that has been written for this in the web page in question so that we can see that acting outside the browser window is a definite requirement to test what has been written,and so that someone might be able to make suitable alternative suggestions for your needs.
However, just as webdriver cannot act outside the browser window, so far as I am aware, neither can the code in a web page (without gaining your explicit permission to do so) so I am not convinced yet that you really need anything more than sending the file path to the html input element.
If you can't provide more detail then you might want to look at tools like java robot or sikuli ... but you might just be expending effort unnecessasarily there.