Well... if the attachment comes through when you send it to yourself, then
it's definitely working. The problem would appear to be on the receiving
end. You might ask them "What the heck are you doing with my
attachments!?!!?" ;-)
But, seriously... there are some serious mega-worms and viruses going
around, and a number of email servers' administrators have instituted
Draconian measures to try to stop them. I'm seeing all kinds of seemingly
tame email being bounced, and attachments stripped off. It's possible that
the attachments, simply by chance, contain strings that "look" to their
filters to contain items that are being screened out.
If you have web space available, an alternative would be to upload your
attachments to your web space, and then send your client(s) the links. That
way, they'd be able to get at them even if email filters (over which they
might not have any control) are overly restrictive.
Another alternative would be to zip the attachments first. In zipped form,
perhaps they'd get passed whatever filters are being used.