Comment 60 for bug 1776873

Nevermind my last comment. This works in chromium (no idea about snapd).
To get this to work in chromium, first make sure it works for xdg-open.

Click on the link that you want to be opened automatically in some
application. When it downloaded, right-click on it (the button that
appeared on the bottom) and select "Always open files of this type";
this will cause chromium to use xdg-open next time you open a file
of that type.

To make xdg-open work, run "xdg-mime query filetype the-downloaded-file",
this should print the correct mime type.
Then run "xdg-mime query default that-mime-type", this should print
the desktop file that you want to use, aka "myapplication.desktop".
Inside that .desktop then (.local/share/applications/the-file.desktop),
there should be the lines Exec=/path/your/application and MimeType=that-mime-type
Anyway, plenty of tutorial on xdg on the net. I missed the chromium
trick to make it use xdg-open in the first place.