Hochladen ohne Datei auszuwählen wird nicht abgefangen

Wähle ein Protokoll aus, klicke auf Editieren, klicke auf Hochladen ohne eine Datei auszuwählen, erhalte einen Internal Server Error. Im Log sieht das dann so aus:

[2018-07-09 14:54:35,335] ERROR in app: Exception on /protocol/upload/known/3 [POST]
Traceback (most recent call last):
  File "/var/www/asta-protokollsystem/program/lib/python3.5/site-packages/flask/app.py", line 1982, in wsgi_app
    response = self.full_dispatch_request()
  File "/var/www/asta-protokollsystem/program/lib/python3.5/site-packages/flask/app.py", line 1614, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/var/www/asta-protokollsystem/program/lib/python3.5/site-packages/flask/app.py", line 1517, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/var/www/asta-protokollsystem/program/lib/python3.5/site-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/var/www/asta-protokollsystem/program/lib/python3.5/site-packages/flask/app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "/var/www/asta-protokollsystem/program/lib/python3.5/site-packages/flask/app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "./shared.py", line 154, in decorated_function
    return function(*args, **kwargs)
  File "./common/database.py", line 40, in _decorated_function
    return function(*args, **kwargs)
  File "./decorators.py", line 42, in _decorated_function
    return function(*args, **kwargs)
  File "/var/www/asta-protokollsystem/program/server.py", line 731, in upload_source_to_known_protocol
    if file.filename == "":
AttributeError: 'str' object has no attribute 'filename'