How Do You Find a File by Name in GitHub (when it is not otherwise showing up)?

Problem scenario
You are searching GitHub for a file by its name. But for some reason it is not showing up in the results. What should you do?

Solution
Use the "filename:" option in the search terms before the name of the file you are looking for.
org:coolorgname filename:rio.yml

For some reason this "filename:" option returns more results. (This solution is supported by https://stackoverflow.com/questions/18991908/is-it-possible-to-search-for-a-particular-filename-on-github.)