if the original packet already contains a file name,
don't add a generated tree item with the filename stored in the
conversation

Change-Id: Ia9a7fe712d0dd8241399e456509019b8e3902554
Reviewed-on: https://code.wireshark.org/review/3447
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>