mhstore is not decoding the filename:
$ mhstore -auto -part 2 storing message 3677 part 2 as file
[...]/=?UTF-8?b?Z21ncCBjb2NoYWlyIGR1dGllcy5kb2M=?=
Unfortunately, RFC 2047 encoding is explicitly forbidden to be used for
MIME parameter values. There is a different standard for encoding MIME
parameter values (RFC 2231) that nmh supports. But it seems that a lot
of MUAs still get this wrong. I'd rather leave it that mhfixmsg will
correct this instead of natively decoding it.
--Ken
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