I'm not a script guru like @jesus2099 .
Until he find some time to write a really flexible and friendly Userscript, I use this workaround:
1.) Copy & paste this line of code as ONE LINE in the adressbar of your browser (tested with FireFox):
2.) Create a bookmark for this single line of code ***
3.) Use this bookmark on the MB /add-cover-art page with just freshly uploaded covers and it will mark every occurence of the "Booklet" type checkbox for every uploaded picture.
4.) You can easily change the type of checkbox to be activated by changing the number follwing "value="
The # 1 is the Front Cover, # 2 the Back Cover, # 3 the Booklet and so on up to 13 for Watermark.
*** if you get a warning about «...preventing social engineering attacks...» you may have installed NoScript. It's up to you to create the bookmark directly or deactivate NoScript for this step.
If you see your bookmark with the above line, you can re-activate NoScript and the line works fine.