Steps to reproduce the issue

Go to the template manager and select one of the templates
On the toolbar select new file and create a new file eg brian.php
Now select the newly created file and on the toolbar select rename file and try to give it a new name eg simon.php

Expected result

File is renamed to simon.php

Actual result

Invalid file name. Please choose a file name containing a-z, A-Z, 0-9, - and _.

System information (as much as possible)

Tested on 3.6.2 and current staging