Fixed issue where newly attached items get a `SortOrder` value of `0` (this was because `attachFile` is no longer available in the base-class and therefore never gets called).
Changed `SortOrder` to start at `1` and not at `0` to be more consistent with other sort-rows in the framework. This also fixes an issue in `Sortable.php` where a value of `0` falsely got the max-value assigned.
Attaching and sorting of files now works on new (eg. unsaved) DataObjects too.