So, a large amount of the media in the "files" folder is named incorrectly. it seems you've saved webp files with the both jpg and gif file extension. while this probably won't cause issues because ***MOST*** browsers don't care about file extension, it would still be better if you were to name them based on the actual encoding used so that the few that do care, or any security software that cares, don't have problems. This will also prevent any issues if someone is using a weird video/image codec setup.
if you want a program that can do (most) if the renaming of the files for you, IrfanView is free and can do that. I did find a few that when renamed caused a file name conflict. Allie.gif is a webp, and allie.jpg is also a webp, however the files are not the same. one is a picture and the other is a video. So you will still need to go through and manually fix those...
and then change the file names in the HTML source.
In case you (or someone reading this post) cares, the reason most browsers can function correctly is because they look at the file header to determine what the file type is, which then loads the appropriate codec to view said media. Some security software will detect the file header/file extension mismatch as a possible attempt to compromise the computer and block it.