After the handler is added we get the following ERROR in the log file.

2011-12-17 18:30:20,387 http-8090-5 ERROR admin 1110x14825x1 17q326h 0:0:0:0:0:0:0:1 /secure/admin/EditHandlerDetailsUsingParams.jspa [service.services.file.FileService] You must specify a valid handler class for the com.atlassian.jira.service.services.file.FileService Service.

The reason is 2-phase process of adding new service by DefaultServiceManager. First the manager adds a service without any parameters, but still it calls init() on the service method (with empty PropertySet). Subsequently the implementation calls:

serviceConfigStore.editServiceConfig(serviceContainer, delay, params);

which calls again init() this time passing valid service/handler params.
Our stronger error checking has to deal with it (i.e. extra call of init() with empty params), ultimately we should though change the way how
ServiceConfigStore works and add there the ability to add service together with params in one call.