Useful debugging tools:
XDG_UTILS_DEBUG_LEVEL=2 xdg-mime query filetype foo.pdf
XDG_UTILS_DEBUG_LEVEL=2 xdg-mime query default application/pdf
fd evince.desktop /
This works:
xdg.mimeApps = {
enable = true;
associations.added = {
"application/pdf" = ["org.gnome.Evince.desktop"];
};
defaultApplications = {
"application/pdf" = ["org.gnome.Evince.desktop"];
};
};