how to make gifs not inline?

Looking at the docs, it appears image inlining is instigated by the
MIMEARGS defaults. I tried to override that for a mail with an
attached .gif and no Content-Disposition: header. It still got
inlined. What am I missing?

Thank  you in advance,
Jeff 
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MHonArc v2.3.3 (Perl 5.00404)

<MIMEArgs>
m2h_external::filter; usename useicon subdir iconurl="../attachment.gif"
image/gif;
</MIMEArgs>