I think you may be missing some headers 

MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=frontier
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset=\"UTF-8\"

[download]
Without multipart/mixed i think it just sees the rest as text.

https://en.wikipedia.org/wiki/MIME#Multipart_messages

Re: Sending An Attachment Using Net::SMTP

In reply to Re: Using Net::SMTP to send email attachments by huck
in thread Using Net::SMTP to send email attachments by astrobal

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