russian message with attachmen - forced into base64

I create message with attachment and it creates as base64.
Message conntain russian text, attachment is zip.

Why do Sylpheed convert text into base64?
Message out: base64 (line lenght 76),
  incoming: base64 (line lenght 72) and sign is BAD :(

In preferences: koi8-r / 8bit.
                ^^^^^^^^^^^^^
-- 
MaxBritov

GnuPG KeyID 0x6F3DB1FB keyserver hkp://keyserver.kjsl.com


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