100 Continue
Go ahead with the rest — the server accepts the body before receiving it.
A client about to upload a large body can send just the headers, with Expect: 100-continue, and wait. If the server answers 100, sending the rest is worth it; if it answers an error, the client saved the megabytes — and the embarrassment of uploading a whole file only to get a 401 at the end.
curl does this on its own for large uploads, which is why almost nobody sees the 100: the tools swallow it. It is interim by definition — after it, the final response always follows, with its own status.
> POST /acervo/upload HTTP/1.1
> Expect: 100-continue
> Content-Length: 104857600
< HTTP/1.1 100 Continue