Fix Private Methods in FileImporter

Fix private methods in `FileImporter` which prevent overriding the
`FileImporter` class without also copying in those methods

- Set `FileImporter::generatePrefix()` to protected
- Set `FileImporter::storeFile()` to protected

http://www.concrete5.org/developers/bugs/5-6-1-2/fileimporter-override-fails-because-of-private-methods/