Android/iOS: Chances are you don't just use one service to store all your important files (and, if you do, you probably shouldn't). Searching them can be a hassle though. FindIt simplifies the process.
FindIt allows you to connect your Gmail, Google Drive and Dropbox accounts and search all of them at once. You can refine your searches by specifying what type of attachment or file you're looking for, when it was made or who sent it to you.