ark steals appimage downaloads and give no option to save "Loading the archive /tmp/arkG24612.2.2.AppImage failed with the following error: Archive corrupted or insufficient permissions." ark should not open appimage file unless instructed too. this need to be removed from the defaults
It happens only when AppImage file has no exec permission. Looks like a DOWNSTREAM Kubuntu issue (at least in Kubuntu 17.10, Plasma 5.10.5 Ark 17.04.3) as ark is registered there for .AppImages files (which is useless). In Neon Git Stable (Plasma 5.12.2, KF 5.43.0, Qt 5.10.0) ark is not registered to open .AppImage files so clicking on it when it has no x permissions just opens a "Choose app" KDE dialog, which is a proper behavior.
...not that showing the "choose app" dialog is really that much better, since there's no app in that list that will do the right thing. Sadly this is just a major inherent usability issue with AppImage. Regardless, Edmund is right: the specific issue here is Kubuntu-specific, so if you'd like to pursue it further, please report it to them.