attachment extensions yet again

I've read the mailing list archives, but I still can't get this to work.  I
want all my attachments to use the extension specified in the MIME headers,
and it worked in 2.4.7.  I upgraded to 2.6.10, and it no longer works, using
the same rcfile.  This is what is in my rcfile:

<MIMEArgs>
application/octet-stream;       subdir usenameext
</MIMEArgs>

When downgrading back to 2.4.7, and running against the same messages, it
works correctly.

How can I get back to the 2.4.7 functionality?

TIA,
Ian