allow File to behave like normal backbone models

we had written some code for the backbone File model
that assumes that you would always have a 'file'
attribute (that is an actual filesytem object representing
the bytes of the file), so you couldn't save
or toJSON it if you were wanting to save it's other
attributes (like name or parent_folder_id)

this just fixes that.

test plan:
make sure that files still work as
intended in quizzes or other places we use files.

Change-Id: I4d9b7283b2d9d7160fd5b95819d9b7093f4c1cf0
Reviewed-on: https://gerrit.instructure.com/39916
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>