Package for classes to handle creation of thumbnails and other transformations and processing of exhibits, selected by type from the (normalised) exhibit file extension.

Some of these may need particular non-standard packages to be available at run-time, so it must not be assumed that once available, always available. For example, we might have precomputed thumbnails available that we could not recreate at the moment.

Note that the base ExhibitMIME.Handler class that classes in this package should all implement is a ``fat'' API, ie implementing classes may not implement all of the API at run-time but return default/failure values instead.