MIME = Multipurpose Internet Mail Extensions.
It is a standard that extends the format of email messages to support text in character sets other than ASCII, as well as attachments of audio, video, images, and application programs.
Courtesy of Wiki.
So is it a setting he needs to change on his phone?
A friend sent me a seven old photos on her phone and I got seven “Removed attachment of banned mime type”. These were personal photos of an old friend. ??