(Will add pictures and details when I find a good way to change extensions in an IPhone)
You can’t, there’s no way to do it. Despite the fact that this request has been made by many users many times over the years, the forum admins here haven’t added the
.jpeg extension (which is actually one of the official extensions for JPEGs, even if
.jpg is far more common) to the list of permitted extensions for attachments.
The only workarounds are to use Tapatalk or some other third-party photo sharing app.