The default behaviour is unchanged, but if you write "yes" to
$:/UploadWithUrlOnly then it will assume it's possible to upload
with a blank username and password, as long as the host is set.
The motivation is to support a upload plugin compatible upload
service that uses some method to authenticate other than the legacy
upload plugin user/password params.
Without this patch, the user would need to enter something random in
the user and password fields for TW to decide the upload plugin can
be used.