In the Column management window you can use both both "%_filename" and "%_FILENAME%" for "Value" and "Sort By", but if you use "%_FILENAME%" for "Field" then it gets blocked- you can no longer change the name of the file in the main window. But you can change the TITLE and then copy it to the _FILENAME with action. So at the same time it is blocked and it is not
And also it does not matter if you put "%_FILENAME_EXT%" or "%_filename_ext%"- they both work the same 100%. So you can have a settings of Columns where changing of the filename is blocked via the "%_FILENAME% but is not restricted via the "%_FILENAME_EXT%". So this is either merely an inconsistency or an obvious bug. [If it was to me, I would deem such blocking of edition of "%_FILENAME" a bug]
And also there is some kind of strange bug. When I started to temper with using small and big letters for those two variant [of showing and sorting of filename or of filename and its extensions] I got a box with an error every time I entered the Columns management window. But that message box displayed nothing: it was empty, only with the OK button. Or was it without even the OK button? I do not remember now: but for sure it had no text and had a form of a small square or almost a square. It showed itself also every time I chose the entry bearing my "%_FILENAME%". But that buggy message did not appear at the very beginning of my tests with those values but only some time [steps] later. I am also unable to repeat its appearance now, but back then it was coming back every time I was using the "%_FILENAME" for "Field" and going away after switching to "%_filename". And then it just stuck itself, no matter what I was using. But it finally disappeared when I used "%_filename" for "Field" and closed and reopened the whole Mp3tag [and not just the Columns management window]. [And just to be sure that this bug did not hide along itself for the ride, I restored my settings from a copy]
[And if you are wondering, what do I need capital letters for inside the %%- for visibility. As there are no colors in Mp3tag, reading long scripts can be hard and generate errors. But sticking to a simple rule that all tag fields and those system information kept inside %% are to be in CAPITALS, makes a very helpful difference]