how about using zimbra_session_limit_imap option in zmlocalconfig?
this option means "Maximum number of IMAP folders a
single user can have open at once"

I'm sorry for wrong information.
I found that -Dsun.nio.cs.map option is only effective at the specific locale (I think this option is only effecvite for Japanese locale.)

actually,　I use the option to change a charset mapping for japanese.
(x-windows-iso2022jp/ISO-2022-JP)

So, I hope it works for other charmaps.

I filled a bug report at

Bug 30622 – cannot download files from briefcase in the case of using multibyte characters as a filename

I remember that sun's jdk has an option to swich charset mapping.
so can we use -Dsun.nio.cs.map=GBK/GB2312 property?

so try to use it

zmlocalconfig -e "mailboxd_java_options=-client -XX:NewRatio=2 -Djava.awt.headless=true -XX:MaxPermSize=128m -XX:SoftRefLRUPolicyMSPerMB=1 -Dmail.mime.decodetext.strict=false...

do the following commands

# su - zimbra
$ zmlocalconfig | grep mailboxd_java_options

then, check it and adding above options

$ zmlocalconfig -e 'mailboxd_java_options=........

can you try to add '-Dmail.mime.decodetext.strict=false -Dmail.mime.decodeparameters=true -Dmail.mime.decodefilename=true' options to mailboxd_java_options?

it seems ok for me.(In Japanese).