When you want to add an image it says:

Supported formats: gif, jpeg, jpg, mp4, png. Max size: 3 MB

However, when you click "Pick image" and navigate to an mp4 image (i.e. an mp4 file without audio) smaller than 3 MB, it does not show up as a supported file type. If you then try to add it anyway, nothing gets uploaded. So it seems that despite being listed as a supported format, mp4 images are not actually supported.

Feel free to move this to Questions & Support if that seems more appropriate, I just stuck it under Feedback because it's more of a bug report than something that needs answering or help, since it isn't hard to convert mp4 to gif.