Sophocle,
That is an excellent idea! The only problem that I saw with it is that perhaps some people prefer that users with less than the minimum # of posts not be able to download images.. And even with the "view images" option set to "no", they would still be able to download images that way.
But there's an easy solution..
Code:
if ($viewattachedimages==1) {
$extension=strtolower(substr(strrchr($attachmentinfo[filename],"."),1));
if (($minpostattach > $bbuserinfo[posts]) && $minpostattach!=0 && $extension!='gif' && $extension!='jpeg' && $extension!='jpg' && $extension!='png') {
eval("standarderror(\"".gettemplate("error_attachmoreposts")."\");");
exit;
}
} else if ($minpostattach > $bbuserinfo[posts] and $minpostattach!=0) {
eval("standarderror(\"".gettemplate("error_attachmoreposts")."\");");
exit;
}
With this code, if "view images" is set to no, users who do not have the minimum # of posts will still get the "not enough post" error message when attempting to download images.. But if "view images" is set to yes, everyone will be able to see the images in the threads, but not download other file types unless they have enough posts.
I've tested this and it works.

I'll update the zip file in the first post.
{edit} I just noticed something.. As an added bonus, this hack will also stop people from hot linking images on other sites if you set view images to no and minimum posts > 0.