An apk disassembly of the Google Play Services application, version 26.32.31 Beta Reveals some interesting details on the backups of the phone. It suggests a new feature that Google will probably implement in the near future that would allow you to choose more files to save.
Everyone using an online automatic backup know that not all data is saved. You get your Cloud photos and videos with settings and some user data, but not everything. For example, some applications have encrypted data and cannot be saved.
Currently, you cannot save other files, such as your “Downloads” file. But the source of beta version for Google Play Services suggests that Google is working on a solution. This will probably give you the opportunity to save your download file, and why not other files too. Of course, all your data will be downloaded from your Google Drive.
In addition, there is a chain that refers to “documents”, which may suggest that Google will allow backups of certain file types and not everything.
Anyway, we can only guess for the moment, but it seems that Google is already working on the new backup options so that we will surely know it early.
