In Exchange, emails sent outside the organization are converted and the format of these can be set per domain or for all domains. If messages are sent in text format, some recipients will have issues viewing attached images.
ContentType should be MimeHtmlText not MimeText when running: Get-RemoteDomain |fl To update it run the following with either a domain or * Set-RemoteDomain -identity "Domain or *" -ContentType MimeHtmlText