Support Whitehall's ImageUploader extension whitelist
Support Whitehall's ImageUploader extension whitelist

This ensure we're setting the correct content type for assets allowed by
Whitehall's `ImageUploader`[1].

I used `Mime::Type.lookup_by_extension` in the Whitehall project to
generate the expected mime types used in these tests.

Note that the `ImageUploader` also allows 'jpg', 'jpeg', 'gif' and 'png'
images but specs were added for these in a previous commit.

[1]: https://github.com/alphagov/whitehall/blob/73f95f72e835503febfdc53db73cdee58d1ae18c/app/uploaders/image_uploader.rb

alphagov/asset-manager,alphagov/asset-manager,alphagov/asset-manager