Hi Andrew,
You can still access the file with {BaseURL}/file?guid={GUID_of_FileDocument}. Perhaps this helps in updating the widget.
For a demo application, we used this URL in combination with the IFrame Component widget to show PDFs in a Mendix application. However, we only used this on localhost and didn't test this for MendixCloud. It is possible this doesn't work in the Cloud unless you allow iframes (doc).
I will share some screenshots of the set-up with IFrame Component, perhaps it helps:
IframeSrc is filled this way:
Hope this helps!