Assume that you compose a message that contains an inline picture in HTML format in a Microsoft Exchange Server 2016 environment. When you delete the inserted inline picture, and then switch the message format to plain text, the deleted picture is displayed as an attachment file.
This issue occurs because the unreferenced inline attachment isn’t removed correctly when this message is switched to plain text format.
To fix this issue, install Cumulative Update 5 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Learn about the terminology that Microsoft uses to describe software updates.