Flutter can't find file downloaded to the flutter_assets folder

Question

I download a file from AWS after runtime, and I checked to confirm that the file gets saved correctly in: /data/user/0/APPNAME/app_flutter/flutter_assets/FILENAME.zip
where APPNAME is the name of my app and FILENAME is the name of the file I store. Then I have a function from a package that needs to access this file, but it has to be in the flutter_assets folder. However, I get a java.io.FileNotFoundException: flutter_assets/FILENAME.zip . I tried adding flutter_assets to the pubspec.yaml file like this:

flutter:
   assets:
    - flutter_assets/

and also added this:

    aaptOptions {
        noCompress 'zip'
    }

to android/app/build.gradle within the android{...} block but it didn't help.