By default, JETI sends html email with mime type multipart/related. This is interpreted and handled correctly by the vast majority of email clients including Outlook, Thunderbird, Gmail, Yahoo, and a lot others but Mail app on iOS.
Mail app on iOS devices does not show attachments from emails with mime type multipart/related. The solution is to configure JETI to change the mime type of the emails to multipart/mixed.
In order to enable multipart/mixed, open the Configuration tab of the JETI admin screen and enable the option as shown below.
Problems with Thunderbird
One problem we have found was that multipart/mixed emails cause problems in Mozilla Thunderbird. So be careful with enabling this option in JETI.