An APK teardown of the Google Play Services app, version 26.32.31 beta reveals some interesting details about phone backups. It hints at a new feature that Google will likely implement in the near future that would let you choose more folders to back up.
Everyone who is using automatic online backup knows that not all data is backed up. You do get your photos and videos up in the cloud along with settings and some user data, but not everything. For instance, some apps have encrypted data and can’t be backed up.
Currently, you can’t back up other folders, like your “Downloads” folder. But the beta version source code for Google Play Services suggests Google is working on a solution. It will likely give you the option to back up your Downloads folder, and why not other folders as well. Of course, all your data will be uploaded to your Google Drive.
Additionally, there’s a string that refers to “documents”, which may suggest Google will allow backups of only certain types of files and not everything.
Either way, we can only guess for now, but it seems that Google is already working on the new backup options so that we will know for sure soon enough.
Source