This is happening to us as well since upgrading to 4.6.12.1. A user is complaining it is happening to him on Chrome. I tried it on both MAC (Safari, Chrome, Firefox etc) and Windows 10 (Chrome) and I wasn't able to replicate it on those browsers. However, I was able to replicate it on Firefox and EDGE on Windows 10.
This occurs when doing a drag and drop. Not when using the "Chose file".