Ok, could be a way, but sorry, this is not really comfortable. You should be able to set the filetype afterwards, not at the beginning. Let's say, you are looking for something and after you started typing you see plenty of different filetypes which contain your term, so instead of limit the results to a specific filetype you have start a new search beginning with a workflow
I'm really astonished, I can't do some basic stuff like this with Alfred