Custom page template with two dots before PHP file extension does not work

If the custom template has two dots in front of the PHP extension it does not work although is available in template menu on the right when creating a page.

1) Create a custom page template like addEvent..php (notice two dots)

2) Create a new page and assign it a newly created template (it will show up)

3) Save the post, check the page - and the page shows default template regardless to what we have just created.

Although the template shows up in a list, it does not work because of two dots in a file name. If we rename it to addEvent.php, then it works like a charm.