NEW 'Choose another file' button on upload field AAS-917

When you have a has_one relation with a file added, there's not an obvious way of swapping one file for the other.  Right now you can remove a file and add a new one, but user's have been confused to this to the point of wondering why there isn't a replace function.

It's particularly useful if you are using SilverStripe on a site where "upload from your computer" has been disabled for a class of users.

Thanks to Jean-Fabien for actually making the change, I'm just the messenger. ;-)