Steps to reproduce the issue

1. Go to templates
2. Click on Cassiopeia (Column heading: Template) to open template files in template manager
3. Click on css
4. Click on offline.css
5. Click on the New File button from the toolbar
6. Enter the Copied File Name as override.css

Expected result

The copied file should be get created.

Actual result

It throwing error invalid file name...

Additional comments

This works fine if we don't add filename without the file extension.