The problem has already been reported in the forum under https://tiki.org/forumthread66743-Newsletter-Sent-Attachment?topics_offset=2
I reproduced it on the demo system for release 18.
Open existing newsletter in demo site
attach a simple .png file
clicked Send Newsletter
Next page shows review of newsletter to be sent, includes "Files" with bullet point
Next page shows "Newsletter successfully sent to 1 users."
The newsletter is received, but no attachment
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