If you still have the "original" Name in a tag like TITLE you could recreate the filename with it.
For example you could use the Convert Tag -> Filename with the Format string: %TITLE%
If you only want to rename your files - and not use the tags - I would suggest to use a specialized file rename software.
If you insist to use Mp3tag for this task, you can have a look at the function$cutRight and remove your 28 characters from the _FILENAME with this Format string: $cutRight(%_FILENAME%,28)