bring media record processing in-line with current (revised) community standards

[danstoner]

tdwg/ac#171

The standard now includes:

In cases where the provided Service Access Point URL includes a standard file extension from which the format can be inferred

So, for example, in cases where media ends in .JPG we should infer it is a jpeg and not require a MIME type.

There are possibly other changes that would be needed to bring our AC processing into alignment with the new version of the standard.

[danstoner]

This would help with situations such as:

https://redmine.idigbio.org/issues/3682

[danstoner]

We probably ought to set a new DQ flag if we add or "guess" a media MIME type. I'm not sure that is actually possible because media processing and indexing are two different processes.

We do already have some support functions in place such as sniff_mime(content) and MediaObject.detected_mime.