Documents tests fail to close a file handle.

One-sentence description

Documents tests fail to close a file handle.

Current behaviour

./home/thom/git/thalia/concrexit/.tox/py36-django20/lib/python3.6/site-packages/django/db/models/query.py:195: ResourceWarning: unclosed file <_io.FileIO name='/home/thom/git/thalia/concrexit/website/media/documents/file_nl_CwarPQj.pdf' mode='rb' closefd=True>

Expected behaviour

No warning

Steps to reproduce

  1. tox
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