Thanks Matt !
That did the trick. I had .iso and .avi.
What is strange though is that Media type is populated with "Unknown".
Maybe it is because I forgot to untick "Image" => JRiver might not have been able to define what king of media it has imported (as I had ticked Image, video and data).
Anyway, it's fixed, thanks again !
This piece of software is just amazing
And great support also !
Cheers,