Providing a focus style for Browse buttons

See #2723

The Browse button in the upload widget doesn't have a focus style when tabbing onto it. It can be used using the keyboard only, but you can't tell when it's focussed.

The solution in #2723 doesn't work as there are security constraints around faking a click on a file input field, plus we also lose the extra handling that screenreaders do on file input fields. Therefore, we just show a fake focus style on the button.