Tried this, but it gives a 404 error:

return send_file( '/dev/null',
        streaming    => 1,
        system_path  => 1,
        content_type => $mime_type,
        callbacks    => {
            override => sub {
                my ($respond, $response) = @_;
                my $writer = $respond->([ 200, {} ]);

                my $buf = '';
                while( read( $in_fh, $buf, VID_READ_LENGTH ) ) {
                    $writer->write( $buf );
                }
            }
        },
    );
[download]

Debugger still shows that the callback isn't called.

I do believe that the docs for Dancer2 send_file are more "wishlist" or "todo" than "this is how it works", this is based of my reading of send_file and Dancer2::Handler::File

send the bug upstream :)