Bug: failure of _FILENAME behaviour and docs to accord
1 See Help C:/Program%20Files/Mp3tag/help/options_format.html
"Note: Actions applied to _FILENAME are applied to the whole file name including its extension."
2 On a file xxx.mp4, run an Action using FORMAT VALUE to set _FILENAME to xxx.mp3
Expected: filename xxx.mp3
Observed: filename xxx.mp3.mp4
Note: On V2.49, Help is modified to accord with behaviour.