Can't Attach Photo From Photos

Hi,

When I try to attach a picture out of my iPhone to a post like this I am able to see the pictures but they cannot be selected. They seem to have a strange extension of HEIC. Why do these pictures have this extension and if Apple cannot use them in Apple forum then what are they good for?

Is my phone setup incorrectly? I use the "Image Insertion" at the bottom then it opens a Finder window and I select Photos as the source.

Thanks

Farzad