While handling attachments upas/marshal first looks at the
file extension which it matches against a built-in list and
then against the system mimelist.

Both of those currently contain only single-component
extensions such as '.tgz' but marshal uses anything after the
first '.' in the name as an extension, probably in
anticipation for '.tar.gz' and similar.

This patch changes marshal to match the last '.' delimited
component.