Windows Explorer FTP client expected a success response to "OPTS UTF8 ON" 
command [1], otherwise it will assume the server doesn't support UTF-8 and 
display UTF-8 encoded filenames incorrectly.

This OPTS command isn't supported by pyftpdlib [2].

I've got a report in ftpclodfs related to this [3] and the workaround was 
simple: just return success to a "OPTS UTF8 ON" request [4].

I don't know if this is just windows not playing nice with standards but I find 
that implementing the OPTS command is harmless and fixes UTF-8 support in 
windows' default FTP client.

References:

 - [1]: https://tools.ietf.org/html/draft-ietf-ftpext-utf-8-option-00 - [2]: 
https://code.google.com/p/pyftpdlib/wiki/RFCsCompliance#RFC-2389_-_Feature_negotiation_mechanism_for_the_File_Transfer_P
 - [3]: https://github.com/chmouel/ftp-cloudfs/issues/50 - [4]: 
https://github.com/chmouel/ftp-cloudfs/commit/b4bef39fd394c147c2a85c4f25758bb1b14a9d1f