In order to search Google Groups, you need simply to be signed in with Google Account otherwise you may encounter no results or various errors like:
There was an error performing the search. Please try again later.
Once you are logged in, you are able to search it as usual Google Search pages:
And as regular Google Search, it supports all ...
Other have already pointed the pdf, I'm pointing here a more complete article from Google support with all the filetypes available to use with the filetype: operator
It's a small article so I will paste here for ...