First of all, the icons to include in
aces-dev should be provided, for each of the three file types (ACESclip, CLF and CTL), in all the file formats directly usable by the three main operating systems:
.icns for macOS;
.ico for Windows (most Linux window managers accept it as well);
.png for compatibility with all Linux distros.
The first two formats package icon sizes together in a single file; for the latter, one PNG is required per icon size.
Windows and most Linux OSs (particularly Linuxes based on KDE and Gnome window managers) both have one registry of “icons vs file-types” associations, at system level.
For Windows, in particular, this can be packaged putting the customized registry modifications in one
.reg file that can be just double-clicked upon to implement the new associations (as simply as @Alexander_Forsythe asked for)‡.
With macOS, it’s a lot more complicated instead.
To make it short, as @frankjonen says, the easiest way is providing an application (to be installed in
/Applications folder) that becomes the default one for opening the three files types. The
.icns files would simply be included within its files to induce the icon associations. But what would the application do, as it would be called whenever a user double-clicks on a
The OS-agnostic script from the previous reply can implement the file-type registry modifications (or the macOS app installation)
‡ Some anti-virus programs may not allow this.