NEXUS-6109: Rework of MimeSupport implementation.

Figured that Tika class offers high level access to MIME
detection, and is not what we want. We need "deeper" access
to underlying actual MimeType instances, and, more importantly,
we want to "unravel" all the super types all way up to root
mime type, that is needed to perform proper content validation.
Also, there is a twist with a Nexus specific case: empty XML
files should be banned. Hence, Tika detection must be used but
without name metadata, that would mislead Tika.
NEXUS-6109: Handling the "empty XML" edge case

Ny using the newly introduced methods on MIME Support
NEXUS-6109: Adjusting UTs

As now, root MIME type (octet-stream) is always returned
as "least specific" MIME type (basically, whatever content).
Also, slight internality change about use of NexusMimeTypes
caused to change how mock reacts too.