Add content_type to testartifact file storage

Summary:
    So that the browser gets it and displays the file based on content.
    Added a method get_content_type to filestorage mock to test it.
    Added the same method to the real storage for consistency (not called anywhere and not sure how to test that).

Reviewers: vishal

Reviewed By: vishal

Differential Revision: https://tails.corp.dropbox.com/D80527