Work around bug in filemapper?

This is a workaround that will solve an issue I ran into and it should also fix #64 and #66. This might be a bug in filemapper, but I'm not sure. It seems like `select_file` gets called one too many times. I dropped a debug line in this method and I see the expected values of the resource `interface` plus one extra call where `interface` is nil. When `interface` is nil, a partial file path gets passed back. If we return nil from the whole method everything is happy.