This is a feature request.
After executing C-c g [keywords], I'd like to narrow the search result by file name or extension.
How about providing a command (bound C-c u for example) to switch the search-scope from files to grep-buffers?
C-c g [keywords] C-c u [filename] C-c u [keywords]
(I know I can save the grep result and search there, but doing it at once is far better!)