The filesystem where I have installed Roundcube is perfectly capable of storing/displaying UTF-8 filenames.
When I have a look at what appears on the filesystem, I get the admin eye-cancer.
drwx------ 6 vmail vmail 1024 Feb 16 17:34 '.&AQw-LK'
drwx------ 6 vmail vmail 1024 Feb 16 17:40 '.&AWA-koln&AO0- j&AO0-delna-Z&AWA-BH'
drwx------ 6 vmail vmail 1024 Feb 16 17:45 '.Bo&AX4A7Q-darsk&AOE-'
drwx------ 6 vmail vmail 1024 Mar 5 16:28 '.byt &AWA-im&AW8-nkova'
Is there any config file to tell Roundcube (is it in Roundcubes' responsibility anyway?) to simply use UTF-8 and not to mangle filenames with some knee-jerk encoding?
This is IMAP server's responsibility.