This has to do with sending content-disposition in older browsers, also see http://greenbytes.de/tech/tc2231/ That said, you could file a ticket and maybe we can have a look at this again and see if we can improve.
I'm not sure but what happens if you set an after commit action that checks whether the name contains %20 and then replaces that with a space? But it might be worth a try.
To be a bit more specific our issue is located in our email exchange integration. I did some more research and the issue seems to be caused by the integration (java code). Thanks for your assistance for now.