[Android] Open the document app on the phone to pick a picture (or any kind of files)

ChoosePicture.java

    private static final int PICK_IMAGE = 1; // number that identify the request (can be any number)
    //...
    
    // Called e.g. when clicking on a button
    public void addPicture() {
        Intent intent = new Intent();
        intent.setType("image/*"); // the type of data we are interested of
        intent.setAction(Intent.ACTION_GET_CONTENT);
        startActivityForResult(Intent.createChooser(intent, "Select picture"), PICK_IMAGE);
    }