Since /etc/mailcap is underdocumented and MUAs can't set MIME types properly, people hack their way around it (<a href="http://www.spocom.com/users/gjohnson/mutt/">mutt_octet_view</a> <a href="http://www.davep.org/mutt/">mutt_octet_filter</a>). Unfortunately, this just re-solves the problem that AFAIK, /etc/mailcap was supposed to solve. 

<p>
The best documentation I found was a <a href="http://www.die.net/doc/linux/man/man4/mailcap.4.html">man page</a> online (not on my gentoo box, even though I have an /etc/mailcap). If there's better, please let me know. 
</p>

<p>Caveats include: <b>%s</b> is different in the commmand section than in the test section (in command it's an actual file's name, but without quotes around it; in the test section, it's the name of the attachment with single quotes around it; you can see this in the first octet-stream below). Anyone have any others? 

<div class='source'>
# patrick's mailcap (I know it's missing the proper mime-types for M$)
#
application/octet-stream; oocalc2    '%s'; test=perl -e "%s !~ /\.xls$/i and exit 1" 
application/octet-stream; oowriter2  '%s'; test=perl -e "%s !~ /\.doc$/i and exit 1" 
application/octet-stream; ooimpress2 '%s'; test=perl -e "%s !~ /\.ppt$/i and exit 1" 
#
application/x-pdf; xpdf '%s';
application/octet-stream; xpdf '%s'; test=perl -e "%s !~ /pdf$/i and exit 1" 
#
image/*; display '%s'
#
audio/basic; /usr/lib/mime/playaudio '%s'; nametemplate=%s.au
#
application/x-troff-man; /usr/bin/nroff -mandoc -Tlatin1; copiousoutput
#
text/plain; less '%s'; needsterminal
text/richtext; oowriter2 '%s'; description="Richtext" 
text/enriched; oowriter2 '%s'; description="Enriched Text"
text/html; firefox '%s'; needsterminal; description=HTML Text; nametemplate=%s.html
text/*; less '%s'; needsterminal
#
message/partial; showpartial '%s' %{id} %{number} %{total}
message/external-body; showexternal '%s' %{access-type} %{name} %{site} %{directory} %{mode} %{server}; needsterminal
#
application/x-tar; /bin/tar tvf -; copiousoutput
application/x-gtar; /bin/tar tvzf -; copiousoutput

</div>