It might be that you need to have a version of Quicktime on your computer for the mov to work in Studio. A while back there was a lot of talk about issues with QT players being prone to viruses and Apple quit supporting it. I have version 7.7.6 on my computer from 2014 and haven't had any problems myself.

I can give you a copy of 7.7.6 if you like. It's an exe file about 41MB in size that I can put in a zip file. Of course I can't guarantee if it will work for you the way it works for me.

The source for the free Unmult looks okay to me. You do realize you need After Effects for Unmult? It doesn't work in Studio.

Joe

PS there's also a way to make a black and white alpha video in the wmv format but I won't get into that one just yet

 PPS If you plan on using a lot of those particle type files, you might want to take a look at the free version of Particle Illusion by Boris FX.

https://borisfx.com/news/now-available-particle-illusion-standalone-it-s-free/