The FileImporter extensions uses many essential methods from core that deal with more or less complex matters as importing of text and file revisions, adding log entries, using external user names etc. - Changes in core to these components, or the way they work can break the extensions and will potentially lead to broken imports on Commons
The FileImporter extension does mainly contain PHP unit tests but also a handful of integration tests that check the integration with core for the import step itself.
It would be good to run the FileImporter as part of the gated extensions, what then could also avoid things like T220713: Fix broken FileImporter unit test to happen.
- We should make sure that expensive API requests during testing should be either skipped or excluded when run as gated to not further slow down the testing chain ( to be investigated here )
- See T223017: Isolate FileImporter integration test from external web services for above ( not a blocker )