Upload: Fix the final file name collision test in `wp_unique_filename()` when...

Upload: Fix the final file name collision test in `wp_unique_filename()` when uploading a file with upper case extension. Add a unit test to catch that in the future.

Fixes #48975 for trunk.

Built from https://develop.svn.wordpress.org/trunk@46966


git-svn-id: https://core.svn.wordpress.org/trunk@46766 1a063a9b-81f0-0310-95a4-ce76da25c4cd