How to rename uploaded files Using Drupal 8, I want to rename and clean all filenames, e.g. If someone uploads file named like this Screen Shot 2016-03-29 at 1.13.21 PM.jpg, I want to clean it so that convert all spaces to dashes - and make everything lowercase, So I should get screen-shot-2016-03-29-at-1.13.21-pm.jpg.

I want to do this on image upload dialog form of CKEditor.
Curently is not possible using core to transliterate uploaded filenames (referer to this and this core issues). To solve this you need to use the Transliterate filenames for uploads module. Check the comments from above.