MP4 File type not supported

I'm trying to get started with the Studio Subtitling app. I've installed it but when I try to import my first video (MP4) and start testing it, I get a pop-up-message that says the file format is not supported.
When I go to Options> File types and click on the link called "Additional Installed File types exist", It does not suggest any of the following filetypes WebVTT, STL -Spruce and SBV- Youtube or MP4.

Any suggestions on what to do?