Add a hook to allow plugins to download attachments

[lwcorp]

viewinbrowser relies on dl.php to download attachments, which requires `&uid=$uid. While this makes perfect sense for e-mail, it cripples the notion of attachments in archives. I mean, there's no sense to see an archive if the whole purpose of the mail was its attachments.

Could you find a way to bypass the protection for certain cases? It could be a special exception for this specific plugin, like changing:
if (isset($_GET['uid']))
and
if (empty($userid))
to
if (...|| (viewinbrowser is used && getConfig('viewbrowser_anonymous')) && getConfig('viewbrowser_anonymous_attachments'))))

Or maybe a more general approach.