With the Filesystem Browser side panel plugin enabled in Kate, I can RMB-click on a file and get an "Open With" menu item. The contents of the submenu lists (for example, on my system): "KWrite", "Okular", "Other...". When I select "KWrite" or "Okular", it still opens the application chooser, as if I had selected "Other...". This is a minor regression compared to some older versions, and could be a KIO widgets bug. Versions: KF5 git of Frameworks and Apps.
Most likely a KIO bug, Kate mostly just reuses KIO widgets and plugs it into a layout. Even saving / restoring the config is in the hands of KIO
Some commit fixed this.