Command for managing album art filenames

This has been causing a bit of a headache for me. Originally, I made the mistake of using artwork.jpg as my standard album art image name. I changed it to folder.jpg by mass-renaming instead of changing my config.yml and then updating.

I finally managed to get everything back and stored as folder.jpg by renaming on a case by case basis so that the script was happy about files that used to be 'missing' (renamed).

Now the issue becomes I need to figure out how to force usage of folder.jpg and folder.jpg only. I absolutely hate having others like folder.jpeg, or folder.png. There should be an option to convert to jpg and force lower case on the file name (this isn't often an issue but I've once or twice downloaded something that had an uppercase file extension).

Beets also seems to have some of the files stuck with folder.1.jpg. If I rename it to folder.jpg, the next time I try and do anything with that particular album, it's going to whine and tell me that folder.1.jpg isn't there. Then I'm going to have to go back, rename it, and make beets happy again.

Are there any easy ways to solve this issue that I'm not seeing?

Thanks!