Most of the phone which comes with GMS services pre-installed have Google Photos doing image backups and editing pictures with various kind of tools provided by the app. 

While Files by Google app is completely unrelated to Google Photos, Google is now testing a floating action button in image preview screen of Google Files app to make image editing a little bit easier. This can save you some time when you want to quickly edit any image while browsing inside the Google Files app. This applies to images stored in your internal or external storages only. 

This new experiment uses the Google Photos' image editing tool. So it depends on the Google Photos for image editing and don't have its own editing interface. But the kind of way Google implemented it, makes it feel like it's part of the Google Files app itself. If you don't have Google Photos installed, the button will not show up in image preview screen.

When viewing images in Google Files app, you will see a floating editing button on the bottom right corner. Tapping on it brings the image editing interface of Google Photos and when you are done editing, tap on "save copy" button and it will bring you back to the image preview screen. This happens so seamlessly you won't notice that you just edited your image in Google Photos. Here is a screen recording of the editing flow - 

The feature is currently hidden behind a flag and is not accessible for users right now to flip with root. So Google may bring additional improvement when they decide to bring it to public. 

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