I have this problem with the latest version of iOS plex app (v 6.11) and my iPhone 11. It upload screenshots as normal, but ALL videos and photos that gets uploaded gets the .plist extension. The two .plist files on the screenshot below are videos and images taken on my iPhone 11
It seems like any IOS photo that has been edited in anyway shows up in Plex as .plist. If you “rename” and change the file extension to JPG it will revert back to a photo and show in library…