You could do that by creating categories, but it would not be automatic.
There is no ExecOnCompletionCommand, as far as I know. I have:
[UserEvents]
[UserEvents/DownloadCompleted]
CoreEnabled=1
CoreCommand=doneDL.sh "%NAME" "%FILE" %HASH %SIZE "%DLACTIVETIME"
GUIEnabled=1
GUICommand=kdialog --title "aMule Download Completed" --passivepopup "%NAME" 10
doneDL.sh is a script I wrote which is on the path. You can easily play with bash and filter by extension. If you wish, please post your work here.
Cheers!