1.13.x branch. The cover uses .jpg, but avatar uses .jpeg. I think both should be valid.
Uploads. No subdirectories for uploaded images
When an image is uploaded by a user, it is placed in one folder /public/uploads/files.
However, a folder can often contain a limited number of files and directories, say 64,000. And searching for a file is slower when there are many files in a directory.
Usually, uploaded files are placed into subdirectories: