The problem is only in DeskApp.
Copy an image directly from an external app and paste it in a new email as inline attachement. (without saving the image to disk)
Sending the email is working.
Viewing of the (received) mail with the image is only possible in webapp and deskapp.
In iOS Devices you can read the text but the image is broken (only mime-attachement)
With normal attachement or if you drag and drop the image from disk as inline attachement there are no problems.
I´ve tried to “save as” the image while composing the mail.
I could see that in WebApp the image extension is (.png) and in DeskApp the extension is (.txt)
DeskApp -> 1.9.7 (also tested 1.9.5 and 1.9.0)