Not possible to change filename w/ extension via file view

Even if you set 'Field' parameter of a column to %_filename_ext%, renaming still modifies part before the extension.

I wouldn't call this a bug as the extension determines how the file is treated. Or: a text does become a picture simply by changing ".txt" to ".jpg".
Also, renaming the extention could lead to the fact that mp3tag does not display that file any more.

The file can be renamed. Please note that you have to enter %_filename% as value and as field.
If you leave Field empty then the display is read-only.

Basically the content of a read-only pseudo tag-field, like %_filename_ext%, cannot be changed by the user.

Changing the content of %_filename_ext% from within a list view cell with consequence to the real filename should not be accepted anyway by Mp3tag - and indeed it is not accepted.
Mp3tag opens only an edit area just for the filename part.
This restricted behaviour should be mentioned in the manual.

If you want to change parts of the filename, also filename extension, then use the action "Replace using Regular Expression" on the pseudo tag-field "_FILENAME".

DD.20110607.1810.CEST

There is only one action in Mp3tag which can do this:

Action: Replace with regular expression
Field: _FILENAME
Regular expression: .tmp$
Replace matches with: .mp3

This example is a action I have stored which chages .tmp files into .mp3 files.

I'm not searching for a workaround. I know I can do this with an action. I've reported this as I considered it a bug/oversight.

Why?

In my opinion setting 'Field' to %_filename_ext% should result in modification of filename with extension even though it may cause Mp3tag not see the file (if you change extension to one denoting unsupported file format).

The default setting is %_filename% so it wouldn't cause any trouble for those not fiddling with their column settings.

That's right. Another inconsistency. If "Replace with regular expression" applies regex to the whole filename than why "Format value" should act different?

I think the best solution would be to provide two pseudo tag-fields _FILENAME and _FILENAME_EXT in the future.