I know how to enable multibyte characters in URL segments (via the configuration of the URLSegment class), but doing that has no effect on uploaded file names. If they contain multibyte characters, they will be transliterated. For example, an image file name Pääsiäinen.jpg (a Finnish word meaning Easter) becomes Paeaesiaeinen.jpg, because ä is not simply transliterated to a, but to ae instead. The current result is just really hard to read :D.
So it would be best to be able to keep the file name intact, but another good solution would be to modify the transliteration process so that ä would become a.
Thanks for your support! :)