If you call up a file it has to be done with the appropriate filename or otherwise the related OS function will return something like "File not found".
So either the part in brackets is the same for all files or you have to get the information from somewhere else so that MP3tag can assemble a correct filename or you have to rename the photos.
I am afraid that in the end you will have to do exactly that what you don't want to do: renaming.
BUT: if you open a dos command shell window (execute cmd) you can get a list of all jpgs with the command
dir /s /b *.jpg >ff.txt
which stores the list in the text file ff.txt in the current work directory.
With an ordinary text editor you can create a batchjob that renames all jpgs in that list (you have to edit each line though but a good method is, to replace every line-end with the line-end-character plus the string "ren "
The effort, though, is probably not worth it if there are only a few files to be renamed.
But perhaps I am too narrowminded and someone else gets a better idea.
I've been thinking and looking and finally I not found an expression that allows me to ignore the content of the . I don't know if mp3tag has any function that allows me to achieve my goal, but would say no.
Your idea is good. However, I found a program called Flexible Renamer that allows me to do automatic re-name files of any type. This program has an option that allows me to directly delete N number of characters from a particular point in file name text. The extension of the characters to be deleted in my case is 18. So I rename automatically jpg files and then I import directly with mp3Tag!
Although, it would be interesting that mp3Tag allowed to ignore part of filename to avoid these situations. In this way, with a single program could do everything!