The demo below. The same image is uploaded twice.
The first time the name of the file is “écran.png”. At the end of the upload the link is not updated. The file was uploaded (you can check in the original directory).
After changing the name from “écran.png” to “ecran.png”. The upload is working.
It’s a problem for many countries. By default a screen copy file name on a french mac is “Capture d’écran 2015-mm-DD à HH:MM:SS.png”.