It's used by most users, that's what they are saying, it doesn't matter that the file format is obscure.
If you make a video player based on ffmpeg you take input videos. The attacker can share a specially crafted video with the victim. Same kind of attack as sharing word documents.
If you make a video player based on ffmpeg you take input videos. The attacker can share a specially crafted video with the victim. Same kind of attack as sharing word documents.