It doesn't work and the debug message indicates it's looking for "file://fragments/footer fragment: copy". So, it's like it is doing the mapping properly but then for some reason tacking on the default protocol.
Yes, the mapped value is being prefixed by the protocol value. Up to now this "feature" has been retained for the purpose of legacy support, but I think it's now safe for this to be dropped. Mappings were always intended to be static, hard-coded work-arounds, so if anyone needs a protocol prefix, they can add it to their mapping definition.
I will make the change in the source base ready for the forthcoming beta release, but in the meantime, if you would like to hack the code, you should remove the thymol.protocol reference from the line shown here: