Default UploadField to replace files

Honours the $overwriteWarning setting, rather than warning
about overwritten files, and when that warning is acknowledged by the user,
create a new file instead. We need to set this as close to
the Upload->load() call as possible, since it depends
on non-deterministic config state.
Merge pull request #2603 from chillu/pulls/htmleditor-upload-replace

Default UploadField to replace files