Pictures with mp4 extension from whatsapp msgstore.db

Hello,

I have an physical UFED extraction from Samsung 9300. When I look at pictures I see a lot of pictures in the form of consecutive frames (caps) from the same video file, each with mp4 extension. And these pictures have a play button icon in the middle. However, they are not videos, they are pictures.

The path of the pictures is whatsapp/Databases/msgstore-dates.db.cyrpt8/msgstoredate.db/filename-.jpg

So these pictures are like caps from video files.

How do you think these pictures are formed? Automatically by the phone or by the whatsapp?