Extensions that create a MimetypeHandler subclass can throw exceptions inside Review Board. The sections of Review Board that call those methods have been sandboxed.
Now when a MimetypeHandler subclass from an extension throws an exception, and Review Board logs the errors. The error logs give enough information to find which method in the MimetypeHandler subclass threw the exception.

Unit tests have been written to make sure that functions from an MimetypeHandler subclass have been called, and when an exception is thrown it gets logged.
The tests fail without the sanboxing, and succeed with it.