I don’t think this can be fixed at the moment. Digikam relies on kio for this operation and kio cannot be bundled in the appimage. I would suggest to file a wish on bugzilla. By the way, I had a fully functional ‘open with’ menu on openSUSE TW KDE and no ‘open with’ menu on kubuntu with the same appimage.
As a workaround you could either use a distribution that has the most recent version of digikam in their repos or compile it yourself (if you are on Debian based distro)
I just discovered, that the “open with” list problem works fine with stock ubuntu 18.04 (so the gnome desktop), what would be ok.
But I can not without the feature “open in filemanger” (which does not work either in the appimage version). So I think you are right, using the appimage version is (what a pity) not the right thing for me.
I know Manjaro/Arch have the latest version, but we are a more deb-family here
So I will check, if there is a kind of “ppa” for ubuntu & Co.
Ok, then I think I am going to give Neon a try. Althought I like Gnome Desktop very much, Digikam is my most central piece of software, so the desktop environment could perhaps be secondary…