MOV videos turned to MP4 files with a black end

Hello, I have a project with branching scenarios where I use mp4 videos followed by a question. When I replace the contemporary video (mp4 ) with the right one (mov), Storyline turns the MOV file to MP4 and adds a black end. It's possible to trim it out but I have a lot of videos so don't you know some "trick" to avoid the automatic conversion from MOV to MP4?

Many thanks!