You have to make the file still executable.
Also, it's a good idea to create a hidden folder in your home directory where to move this file.
mkdir ~/.bin
cd Downloads
ls -pl
chmod + x Name.appimage
mv Name.appimage ~/.bin
And you can double-click to install. If there is a question that you should integrate the Appimage into the desktop, please answer with yes dasn the program also appears in the menu.
I hope this helps.
greez