Presently, file://localhost/ URLs are totally unsupported: is_file_url_or_path
correctly considers them files, but they are undecoded and hence
While the standardization surrounding file: URLs is admittedly vague , AFAIK,
*all* literature, and other implementations, unambiguously demonstrate that
file://localhost/ should be equivalent to file:/// :
- The "File URI specification" explicitly linked to from the xdg-utils homepage 
- RFC 8089 section 1.1
- RFC 1738 section 3.10
- Observed implementations of Windows `start`, macOS `open`, Firefox, Chrome, IE
Proposed fix is in the merge request at https://gitlab.freedesktop.org/xdg/xdg-utils/merge_requests/1 .
merge_request processed, thanks.