@Takayuki. I've been in communication a number of times with support at Dreamhost. I tried the plugin on another test site with a different host and it worked fine. That led me to believe the issue was with DH. After some back and forth with them, during which time I told them that logic (to me) dictated that the issue lay with them and I sent them the email header (they said there was nothing amiss in my logs), I received the following response (my email header is embedded in this response, if that's helpful):
It's logical, but the only other place it would be, would be the mail transport, and it's sending the mail, as it should. Our system doesn't strip attachments at all, so everything on our end is working, as it expected. Since mail is getting sent out, that proves that our system is doing it's job. An attachment is only txt in the body of an email, so nothing is stripping that out at all. Here's the logs showing mail was sent:
Jan 12 21:01:05 mensa postfix/pickup: CA369ECD66: uid=1070569
Jan 12 21:01:05 mensa postfix/cleanup: CA369ECD66:
Jan 12 21:01:05 mensa postfix/qmgr: CA369ECD66:
from=<firstname.lastname@example.org>, size=879, nrcpt=1 (queue active)
Jan 12 21:01:05 mensa postfix/smtp: CA369ECD66:
delays=0/0/0/0.04, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as
Jan 12 21:01:05 mensa postfix/qmgr: CA369ECD66: removed
So that leaves one place, the code. Unfortunately, we can't determine
why your code isn't working, so that's something you'll need to handle on your own.
I've disabled all of my plugins and checked and rechecked all of my settings. I've even changed the test email address I had used to see if that mattered. I wonder if you have any further insights? I apologize for the very long comment!