Simplify JavaScript file upload interface and clean up analytics events

While reviewing analytics events for the doc team, I noticed that we're passing a redundant parameter to the Javascript `FileUploader` class.  We pass in the HTML5 file reference, which includes the content type, as well as a `contentType` string.

* This PR simplifies that interface, so `FileUploader` accepts only the file reference.
* It also simplifies the analytics `openassessment.upload_file` event.  None of these events have yet been generated in production, so now is the time to do this kind of cleanup.

@stephensanchez @lamagnifica please review