It took me some figuring out, but I finally had time to sit down and troubleshoot why my emails were not getting to gmail and some yahoo accounts.
After quite a bit of testing, I did figure out that this module does work like a charm in SMTP mode:
https://github.com/apmuthu/FA24extensions
That, combined with the change to line 116 in the class.mail.inc file, allowed the emails with attachments to get through and the correct naming of the attachments.
Thank you for this post! I had a lot of headaches with customers not getting my attachments last year even though I would always BCC myself to confirm it had indeed been sent.