Hi Elizabeth,
What kind of focus are you looking for? You can't type a filename in the input?
The only focus I can think of is on the button. You can try using the Set Focus action to focus on the button.
If this doesn't work, your only option is using a Javascript Snippet widget.
Enable jQuery and find the class of the button and use the following code to focus it:
$('.your-class').focus()
Or even directly open the PC's File Manager:
$('.your-class').click()
Hope this helps!