Importer: Store Unknown Layer Blobs as Generic Media Types Rather than Failing

Context

The unknown layer blob changes from API: Handle Blob Media Types Consistently (#990 - closed) • Hayley Swimelar • 16.3 • At risk were not applied to the importer.

Problem

Rather than storing an unknown layer blob as a generic blob media type, the import will fail when encountering a layer media type that is not already present in the database.

Solution

We'll need to translate these changes from the API, less the feature flags they've been removed, to the importer: !1339 (diffs).

Impact

Imports will not fail due to unknown layer media types.