fixes to some model tests not related to images or versions
Fix to allow a model to be created with just an image and no content blob

However, the validation and conslidation of params between models and other assets still needs some sorting out and clarifying
updated back-end code for handling retaining existign content-blob ids when creating a new version, and also fixed a couple of tests.

the view side still needs updating to present the content blobs to be retained
Updated tests to reflect moving content_type_from_filename out of assets_common_extension

it is no longer needed to be mixed into the asset. Moved the testcode into upload_handling_test