[web2py] email attachments not seen on Macs

[Bob_in_Comox]

I send email out to 150 club members. No problems with simple emails.
However with attachments only PC users can see and download them,
while Mac uses cannot even see the attachments.

using web2py 1.79.1 on webfaction with webfaction email servers

My code (simplified):

from gluon.tools import Mail
attachments = [Mail.Attachment(path-to-file1, filename=filename1)]

mail = Mail( 'smtp.webfaction.com:587', club-email-address, club-email-
login)
mail.send(
                to = [club-member-addresses],
                subject = subject,
                message = plain-text-message,
                attachments = attachments,
                reply_to = my-email-address)

I do not change any default mail.settings.

If I cannot fix this, my backup option will be to store attachments on
our server, and use links to these in the message instead of
attachments.

Thanks for any suggestions.

Bob