bitox

Thanks for the hint!
It worked for me with the following code in functions.php

function sanitize_filename_on_upload($filename) {
$ext = end(explode('.',$filename));
// Replace all weird characters
$sanitized = preg_replace('/[^a-zA-Z0-9-_.]/','', substr($filename, 0, -(strlen($ext)+1)));
// Replace dots inside filename
$sanitized = str_replace('.','-', $sanitized);
return strtolower($sanitized.'.'.$ext);
}

add_filter('sanitize_file_name', 'sanitize_filename_on_upload', 10);

Cheers!