edit the webassist/file_manipulation/helperphp.php
change line 81:
$baseFileName = preg_replace('/\.[^.]*$/', '', $uploadedFile["name"]);
to:
$baseFileName = preg_replace('/\.[^.]*$/', '', $uploadedFile["name"]);
$baseFileName = str_replace(array("ö","ü","ä","é","è","à"), '', $baseFileName);