We want to set migrated content file in the File module as a main file.
Main files are files with sortorder set to 1. Therefore the file manager
must provide a way to set the sortorder property of the new file record.
The second change allowing filepaths not ending with slash is a helper
as we can easily use dirname() as a value (see the usage in the next
commits). Not providing the dirname() was a bug as the reference to a
file in the HTML text did not respect the migrated file path.
