This model will have all of the data that the view will display. The templates will json-encode the return value from your ArchiveReviewUI.get_js_model_data() method and pass that into the constructor for your model here (which is how data gets from the Python on the server into JS on the client). In this case you'd probably have some kind of tree data representing the archive contents.
The view then takes the data stored in the model and displays it.

Should be using single quotes

These should all be single quotes as well

single quotes in the print statement for strings

Let's make sure that everything after base here stays in alphabetical order.

This should inherit from object

This should inherit from object

I'm sure it's already on your radar, but this is kind of backwards. We should check explicitly for application/x-tar when instantiating the TarArchiveFormat, and the else clause should raise.
Inn fact, we might want to have a dict mapping mimetype to the format class so it can just be something like:

try:
    archiveFormat = self.formats[self.obj.mimetype]
    archive = archiveFormat(self.obj.filename, file_bytes)
    data['archive_hierarchy'] = archive.to_dict()
except KeyError:
    raise NotImplementedError

We can probably leave this out for now. Unless you're planning on using backbone routing to add a bunch of separate URLs that are all served by the same view, we won't need this.