Private application files can be uploaded to Slack via malicious uploader

Hi. I have found an issue which allows to retrieve any files from `/data/data/com.Slack/*` directory. The problem is in exported activity `com.Slack.ui.UploadActivity` which accepts a URI to download files. I see that you've added verification

private static boolean isPrivateFile(Uri uri) {
        return uri.getPathSegments().contains("com.Slack");
    }