There's a movie on IMDB called Marble Hornets, they're possibly making a movie based on the videos. Maybe the answer to who uploaded the video will be in that?
I’ve considered that. I personally would rather MH remain its one standalone series, and maybe Tim mentioning “others” could instead be of reference to other Slenderseries (and also just to convince Alex to not kill him). But yes, the MH guys are indeed involved in the production of a movie set for release this fall, and it is based on the series, so who knows?