When on the Files tab of a repository on GitHub, hit the T key for a filename search interface. Just start typing the name of a file anywhere in the project (in any directory), and possible matches will be displayed interactively. So cool!
Thanks so much! I was going crazy not being able to search for files through the top-right search field (I was trying queries like "file:<FILE-NAME>" to no avail) and was just about to give up.
Glad that helped! :)