HTTP post request payload does not reach server

I must be missing something really obvious, such as a mistyped character or something, but when I submit this HTTP request from PhpStorm and I step-debug the Symfony/3 controller that handles it, $_POST is completly empty:

POST https://example.com/api/v1/documents
Authorization: Bearer <redacted>
Content-Type: multipart/form-data; boundary=WebAppBoundary

--WebAppBoundary
Content-Disposition: form-data; name="file"; filename="Fake document.txt"
Content-Type: text/plain

Hello, World!
Uploaded at Unix time {{$timestamp}}.

--WebAppBoundary--
Content-Disposition: form-data; name="typeClass"

PayrollInstance
--WebAppBoundary
Content-Disposition: form-data; name="typeId"

1231
--WebAppBoundary
Content-Disposition: form-data; name="typeTenant"

tenant2
--WebAppBoundary
Content-Disposition: form-data; name="version"

1
--WebAppBoundary
Content-Disposition: form-data; name="documentTypes[0]"

6
--WebAppBoundary

The most infuriating part is that I used this same script last week and it was working fine. I probably broke something since then, but I can't figure out what.