It seems open and schema extensions are not supported for File resources. It would be very useful to have that, especially as it was something kind of already available in preview in One Drive REST API with custom facets (https://docs.microsoft.com/en-us/onedrive/developer/rest-api/concepts/custom-metadata-facets?view=odsp-graph-online).
Having extensions on file would allow us to store custom metada on a file in the graph.