When using 2.37a I am getting more than just a folder.jpg file when I export cover art to file. When I exported the cover art to a folder.jpg I got the following files in the folder as well:
...and so on
Those other files do not have any extension associated with them, however, when I viewed the files in Windows I discovered they were JPEG files. So it seems Mp3tag is extracting the album art multiple times instead of just once in a folder.jpg file.