TC is inconsistent in deciding where a file name ends and its extension begins.
Example:
- You have a file "Track01.mp3"
- Use Shift-F6 to rename it, enter "EtudeNr.1.*"
Result I get: "EtudeNr.1.3"
As stated here "there's no logical way to do it".
OK - so while applying the file mask for renaming (or copying), the first dot has separated the extension (what I consider confusing - however...).
But if you now use Shift+F6, Shift+F6 to select only the name part of the result, the last dot is used to separate the extension (like I would have expected already while copying:
name = "EtudeNr.1", extension = "3" ).

(preferrably treating the last dot as separator for the extension)