Hello,
if you don't have a field for that in the application group... then you cannot do that with "arsdoc get/query" or at least I don't how to do it... :-D
What you can do, if you are on Unix/Linux, it's to use the command "file" to know which kind of file you are looking at... and then create some scripts to add the extension, and modify the index file.
OR
Create a java program that will simulate the "arsdoc get" and with it, you can extract each file, get the extension directly from the Java API, and do whatever you want with the file and the associated indexes...
Hope that helps a little bit for your problem.