I'm feeling a bit like limiting file types by schemes is not something that our majority use case really cares about, and causes major problems with existing files, see.
If adding this functionality is possible via a separate contrib module, then I think we should rip this out. Limiting file types just based on mime types is the core functionality I think we should focus on.
Thoughts? Am I completely wrong? Tagging this as an alpha blocker as this is something that should be removed prior to alpha1.