We ended up changing $configs{images}{attach} to be a directory.
So it worked like this

package MY::PATH::Config.pm
...... more code ......
$configs{images}{attach} = "/path/to/image/dir/";

and later, in another module

my $img = "$configs{images}{attach}" . "$att->{mime_type}" . ".gif";
<img src=$img>
[download]

Which works, and allows us to pass almost any image type.
We even made a ".gif" file (a copy of our unknown.gif) to
handle blank mime types.

Thanks for all the help everyone.

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