testcases for RecommenderXBlock

I messed things up in the previous PR, and thus I make a new one. I think I should also change cms/djangoapps/contentstore/views/component.py , but I forget how to do it. Do you know how to do that, and could you help me check is there any other thing wrong, @pmitros ?
Since the function we are testing here also checks the `content_type` of a file, such as `request.POST['file'].file.content_type`, I was wondering is there any way to set the `content_type` of `io.StringIO`?