I did some research about this PR, and to understand if the issue was reasonable.

Here's what I've found out:

• Flask raises ValueError when the boundary is not found: https://github.com/pallets/werkzeug/blob/dae7e0d06651e54a74f04e1cf24d806c4e2e9be9/src/werkzeug/formparser.py#L270-L291
• Django raises a custom exception: https://github.com/django/django/blob/abfdb4d7f384fb06ed9b7ca37b548542df7b5dda/django/http/multipartparser.py#L79-L83
• Django also verifies if the boundary is valid with https://github.com/python/cpython/blob/27ee43183437c473725eba00def0ea7647688926/Lib/cgi.py#L991-L997
• After that raise, Django catches that exception and raises a 400: https://github.com/django/django/blob/abfdb4d7f384fb06ed9b7ca37b548542df7b5dda/django/core/handlers/exception.py#L84-L94

On Starlette, we raise KeyError with "boundary key missing". Although this PR follows the issue that was raised, and it's actually an improvement, I think we should follow Django's lead, and also create a 400 response on this case.

Originally posted by @Kludex in #1544 (comment)