I think I have had this error in the past. I believe it came from my using Mircosoft's Notepad to edit the plugins. It's a long shot, but that may be the case.
It isn't that type of problem.
The RIFF part is, mostlikely, a wav file. So the archive got corrupted, either during download or during extraction.
I'm now downloading it to see if FileFront has, by any chance, got corrupted.