You can add an action that will re-lower the file extension using a formula like this:
_FILENAME_EXT <== $lower(%_FILENAME_EXT%)
... but I do not know how to do it in detail .. the summer abstinence put me totally away from Mp3tag scripting.
Hmm, maybe this way works ...
Actiontype 4: Replace with regular expression
Field: _FILENAME
Regular expression: ..*$
Replace matches with: \L$0
DD.20100812.1858.CEST