I got the issue. This is because of the new security feature that was added in Android Oreo, FileProvider which is a subclass of ContentProvider and provides read and write access using temporary access permissions.
It requires some tweaking in AndroidManifest.xml file and requires a new xml file need to be added in res folder.
These terms might not sound familiar to you because these all are native Android terms. I will try to update this post and provide the all new code and files soon. Just give me some time.
In the meantime can you please share your mail ids because is no other way to communicate on Medium . You can write me on firstname.lastname@example.org.
Update : Please read Native Android screenshot/image sharing in Unity using FileProvider