Problem with diacritics in file name

Hi,

there is a problem with Czech letters like ? or í and plugin WP Smush Pro and his "backup" feature.

When image has name "?eho?-?ína.jpg", it makes backup file "eho?-?ína.bak.jpg".
When image has name "?ího?-?ína.jpg", it makes backup file "ho?-?ína.bak.jpg".

I have tested it without any other plugins in the newest version of WordPress 4.7.3.

Thanks,
Jiri