FileService doesn't allow to specify a sub-directory inside the import/mail directory

The FileService is now limited to only look at files inside the JIRA_HOME/import/mail directory.
To be able to distinguish between files related to different projects / issuetypes etc. we need the ability to be able to specify a sub-directory and a handler will only process the files inside this sub-directory. Currently all FileService(s) and handler(s) will look at the same dir and have to work themself which files they have to process.