MailService - fix email structure with multiple attachments

[APaikens]

Outlook has problems sending emails, when there are multiple attachments.

"Expected response code "250" but got code "554", with message "554 5.6.0 Invalid message content when handling MIME exception. IMC53"

Analyzing generated email files found out, that there are problems with structure - old code nests each attachment on top of previous one.

Structure was
Content-Type: multipart/mixed; boundary=Btd1YvhJ ← level 1
Content-Type: multipart/mixed; boundary=rsZ9uNfm ← level 2 (WHY?)
Content-Type: multipart/mixed; boundary=sYDuS8mW ← level 3 (WHY?)
text/html (body)
PDF attachment
other attachment 9012
other attachment 9013

Structure after fix
multipart/mixed; boundary=XXX
text/html (body)
PDF attachment
other attachment 9012
other attachment 9013

After exploration there was conclusion about old solution:
Not strictly invalid per MIME RFC — but result has multipart/mixed inside multipart/mixed

The inner multipart only groups:
HTML body
one PDF
Then another attachment exists outside that group

This structure is redundant and irregular, and Outlook is known to behave unpredictably with nested multipart/mixed blocks like this.

[arhimede]

Outlook has problems sending emails, when there are multiple attachments.

Outlook does not send emails using this library, doh

[arhimede]

Please check the PHP Stan error