Instale o perl-file-mimeinfo e configure-o dessa maneira. Veja o artigo da Arch Wiki em xdg-utils :
If no desktop environment is detected, MIME type detection falls back to using file which—ironically—does not implement the XDG standard. If you want xdg-open to use XDG MIME Applications without a desktop environment, you will need to install perl-file-mimeinfo or switch to one of the resource openers that support XDG MIME Applications.