[ubuntu] Icons for video files

This isn't a question about a problem I am having, but just an informational question. I am currently ruing Ubuntu 16.04 but this question goes further back than that. Just how far I don't know, but it's a way.

Anyway, the question is, how does Nautilus select an icon for video files? I have seen three different icons for video files and I don't really see how the icon is selected. There seem to be three different icons. One is a vertical icon that resembles a frame or two of film, but the frames are orange in color. Then there is another icon that looks like black film with the holes along the edges going vertical, and looks like it shows a frame from the associated video. And then a third icon that looks like the black one just described, but is horizontal with the holes in the film ruing along the top and bottom edges instead of vertically.

I was initially thinking that it was maybe something to do with either the file extension, or the framing of the video itself, but I don't think that is the case. I currently have a folder that has 5 video files in it, and they use two of the three icons. When looking at the audio/video properties for the files, there is no patte. There are two different containers (mp4 and mkv), two different dimensions (1920x802, and 1920x1080), the same codec for all 5, two different frame rates (30 and 24), and varying bit rates. Each of the icons will have both of the different properties.

Any ideas???

Thanks,

Wayne