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:
folder(1)
folder(2)
folder(3)
folder(4)
...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.
Are you sure that the audio file doesn't have multiple covers stored inside the tag?
There was only 1 album art image in those files. I just tested again with a file that Mp3tag said had 1/1 album art images and got 11 files. One folder.jpg and folder(1), folder(2), folder(3), folder(4), folder(5), folder(6), folder(7), folder(8), folder(9), folder(10). Each of the folder(#) files is the same image as the folder.jpg file.
Now Mp3tag does extract a file to a folder.jpg and I can using Windows Explorer search for all the folder() files and delete them but this didn't appear to happen in 2.36 so I figured I'd bring it to somebody's attention.