Pulls/3.2/fix upload behat

Due to the change in behaviour from https://github.com/silverstripe/silverstripe-framework/commit/5f7ebd3c235d663b5ad5c0fbcd3d393d2e893306 in 3.2, we now respect the value of the `Upload.replaceFile` config, rather than forcing it to true if we notify of replacement. Now we instead only show the notification if this config would trigger an overwrite.

The new default behaviour is to rename the file, as it used to be prior to the addition of the overwrite notification.