With latest Winterfell, I'm experiencing the following in Roundcube:
- Create a new appointment titled "Rasen mähen" (note the German umlaut), save it
- Click the appointment, select "Options" and "Send"
- New e-mail screen opens where the attachment is named "Rasenmhen.ics"
Honestly, I would expect "Rasen mähen.ics" as filename, pretty nice UTF-8 encoded ;-)