After attaching an png my message is gone in the viewer

I replyed to an email, using Markdown. Wrote a lot of text. After that I added an png to that mail.
Now all I can see is the attachment, my reply in the Markdown preview is gone. That wouldn't be a big deal.

But after sending the email, I looked in my sent messages. And even there all I see is the attachment but no email at all.

Looking in Apple Mail shows the mail in all her pride :)

I'm using Mailmate Version 1.9.6 (5383) on macOS Sierra.

All the best,
Oliver