When using newsletter with image/file attached, the mail reader was not showing correctly the attached file.
Trying with another mail server, the newsletter was simply deleted by the clam antivirus, with a log saying that the attachment seems to be a virus.

Looking at the RFC for mime multipart emails, it seems that a line break is missing when the class eznewsletter.php format the attached files.

After applying the attached patch, it works wonderfull.