Yes I understand. So the only difference to the DocuView implementation would be that instead of showing the file from the D365FO blob storage, you want to show it from external URL.
So by understanding how the DocuView form is implemented, and using a bit of imagination, you should be able to create your own implementation. There's also a possibility that D365FO form doesn't allow you to embed external content directly. In that case you would have to download the file to D365FO (in x++) and then show it.
Do you think you could be able to start investigating this based on these ideas?