Android : Gradle: Exclude file from Android assets folder

Please, do you know a way how to exclude some file from Android project assets folder before multiple assets folders are merged or during the merge?

For example:

android {
  sourceSets {
    main {
      assets.srcDirs = ['assets1', 'assets2']
      assets.exclude 'assets1/myfile.txt' // does not work
      assets.exclude '**/*.txt' // does not work
      assets.exclude '*.txt' // does not work
    }
  }

  packagingOptions {
    exclude 'assets1/myfile.txt' // does not work
    exclude '**/*.txt' // does not work
    exclude '*.txt' // does not work either
  }
}